refactor(server): remove legacy master-worker scheduler code (phase 2)

[byteayan]

Purpose of the PR

• close [TASK] Refactor: phase 2 cleanup — remove legacy master-worker scheduler dead code #3086

#3082 turned off the pre-PD master-worker scheduler but left the role election wiring, the deprecated config handling and ServerInfoManager's persistence path in place as compatibility scaffolding. This removes them.

Main Changes

Role election (first commit):

• Delete the masterelection classes except GlobalMasterInfo (still used by RedirectFilter, ServerInfoManager, the Gremlin whitelist and examples), StandardRoleListener, and RoleElectionStateMachineTest.
• Drop HugeGraph.roleElectionStateMachine() and its two implementations.
• Remove server.role_election and the server.role.* options, including the RegisterUtil registration and the code in GraphManager and StandardAuthenticator that copied them into every graph config.
• Remove the deprecation warnings refactor(server): disable legacy master-worker scheduler logic #3082 added for task.scheduler_type and server.role_election.
• Remove TaskManager's task-scheduler pool, closeSchedulerTx() and the onAsRoleMaster/Worker no-ops.

Server info (second commit):

• Delete HugeServerInfo.
• Remove ServerInfoManager.init(), heartbeat(), tx(), call() and its db executor, plus the server-info-db-worker pool and the constructor parameter threaded through StandardTaskScheduler, TaskAndResultScheduler and DistributedTaskScheduler.

Where this differs from the plan in the issue:

• The issue says DistributedTaskScheduler uses selfNodeId()/selfIsMaster(). Nothing calls them today; I kept them anyway since ServerInfoManager and TaskScheduler.serverManager() stay.
• There was no initSchemaIfNeeded() call for HugeServerInfo to remove.
• HugeVertex.type() still maps ~server and ~role_data to HugeType.SERVER, now through string literals, so vertices left in existing graphs keep routing to the same table on hstore.
• The two transition tests are kept and renamed instead of deleted. They now check that a graph still opens with task.scheduler_type set, and that GraphManager starts and applies server.role when the removed role election keys are present.

Migration notes (for the release notes)

Java API, source and binary incompatible:

• HugeGraph.roleElectionStateMachine() is removed, together with the types it exposed: RoleElectionStateMachine, StandardRoleElectionStateMachine, RoleListener, StandardRoleListener, StateMachineContext, ClusterRole, ClusterRoleStore, StandardClusterRoleStore, Config, RoleElectionConfig and RoleElectionOptions in org.apache.hugegraph.masterelection. Code compiled against 1.7.0 that calls it fails with NoSuchMethodError or NoClassDefFoundError. Since refactor(server): disable legacy master-worker scheduler logic #3082 the returned state machine was never started, so there is no behavior to port: delete the call. The task scheduler is picked by backend (DistributedTaskScheduler for hstore, StandardTaskScheduler otherwise), and the node role still comes from server.role through GlobalMasterInfo.
• HugeServerInfo is removed, as are ServerInfoManager.init() and heartbeat(), which were no-ops. The ServerInfoManager, StandardTaskScheduler, TaskAndResultScheduler and DistributedTaskScheduler constructors drop their last ExecutorService (server info db executor) parameter; callers pass the remaining arguments unchanged.

Configuration: server.role_election and the server.role.* options are no longer registered. Remove them from rest-server.properties. A config that still sets them starts normally, and HugeConfig logs one "redundant option" warning per key.

I went with notes instead of a deprecated bridge because #3082 set no removal version for these APIs, and a bridge would have to keep the deleted RoleElectionStateMachine type alive only to return a machine nothing starts.

Left for a follow-up (#3245): GlobalMasterInfo.supportElection is never set to true any more, so the redirect branch in RedirectFilter is dead, and GraphTransaction.queryServerInfos() has no callers.

Verifying these changes

• Trivial rework / code cleanup without any test coverage. (No Need)
• Already covered by existing tests, such as (please modify tests here).
• Need tests and can be verified as follows:
  • TaskSchedulerServerInfoTest.testLegacyServerLabelsKeepServerType checks the legacy label mapping in HugeVertex
  • TaskSchedulerServerInfoTest.testGraphManagerIgnoresRemovedRoleElectionOptions and MultiGraphsTest.testOpenGraphWithRemovedTaskSchedulerType cover old config keys
  • Ran locally on JDK 11: the full UnitTestSuite (747 run, 0 failures) and the rocksdb core tests TaskAndResultSchedulerTest, MultiGraphsTest, TaskCoreTest (30 run, 0 failures)

Does this PR potentially affect the following parts?

• Dependencies (add/update license info & regenerate_known_dependencies.sh)
• Modify configurations
• The public API
• Other affects (typed here)
• Nope

"The public API" here is the Java HugeGraph interface losing roleElectionStateMachine(); no REST endpoint changes.

Documentation Status

Select one option and provide the documentation location when applicable.

• Doc - TODO: required documentation is pending; complete it before merging.
• Doc - Done: documentation is included here or linked below.
• Doc - No Need: no user-visible documentation is affected.

Documentation files in this PR or paired hugegraph-doc PR:
Paired hugegraph-doc PR: apache/hugegraph-doc#502 (removes server.role_election and the server.role.* block from content/{en,cn}/docs/config/config-option.md). Please merge both together.

[imbajin]

Blocking: no. Summary: Removing the public Java role-election API breaks consumers that depend on that interface method. Add migration guidance or retain a deprecated bridge through the declared removal version. Evidence: The exact-head diff removes roleElectionStateMachine() from HugeGraph.java and deletes its return type.