feat: implement merge source for compaction

[dkharms]

Description

Yet another pull request of series #336.
This one introduces all buildings blocks required for compaction of fractions.

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[cheb0]

I have a strange feeling that we start to use two terms for same thing (lid and posting). I'd stick to a single term (which is a lid since we don't have much choise). At the same time docs/comments might mention that lids are postings.

Besides, a single posting == a single lid. So, util.Pair[[]byte, []uint32] is techically a posting list. But I'd suggest to stick to lids and rename to TokenLids

[dkharms]

Yeah, what I really meant here is TokenToPostingList, but for brevity I've dropped with part with To because this is Pair. Will rename it.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 52.60664% with 200 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.27%. Comparing base (98c583d) to head (6ee2e2b).

Files with missing lines	Patch %	Lines
compaction/merge.go	0.00%	95 Missing ⚠️
frac/sealed_source.go	0.00%	78 Missing ⚠️
compaction/merge_source.go	89.86%	13 Missing and 9 partials ⚠️
frac/common/info.go	50.00%	1 Missing and 1 partial ⚠️
frac/sealed/token/table_entry.go	50.00%	1 Missing and 1 partial ⚠️
fracmanager/frac_manifest.go	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #413      +/-   ##
==========================================
- Coverage   70.67%   70.27%   -0.41%     
==========================================
  Files         226      229       +3     
  Lines       17611    18009     +398     
==========================================
+ Hits        12447    12655     +208     
- Misses       4238     4418     +180     
- Partials      926      936      +10     

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[github-actions]

🔴 Performance Degradation

Some benchmarks have degraded compared to the previous run.
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Name	Previous	Current	Ratio	Verdict
MergeSource-4	------	5c1152
	NaN B/op	74649824.00 B/op	NaN	🔴
	NaN allocs/op	2010802.00 allocs/op	NaN	🔴
	NaN ns/op	1550018039.00 ns/op	NaN	🔴

[github-actions]

🔴 Performance Degradation

Some benchmarks have degraded compared to the previous run.
Click on Show table button to see full list of degraded benchmarks.

Show table

Name	Previous	Current	Ratio	Verdict
MergeSource-4	------	fbf1fa
	NaN B/op	74650272.00 B/op	NaN	🔴
	NaN allocs/op	2010803.00 allocs/op	NaN	🔴
	NaN ns/op	1532087124.00 ns/op	NaN	🔴