fix(synchronization): correctly cleanup key

[tkan145]

What

Correctly release key

• Create a test script called synchronization.lua inside benchmark folder

local root = require('pl.path').currentdir()
package.path = root .. '/?.lua;' ..
               root .. '/gateway/src/?.lua;' ..
               root .. '/gateway/src/apicast/policy/3scale_batcher/?.lua;' ..
               package.path

local synchronization = require('resty.synchronization').new(1)
local inspect = require 'inspect'

local function foo()
  print('calling foo')
end
local ret, result = synchronization:run("foo", 15, foo)
if not ret then
  print(' error: ', result)
end

print("semaphores: ", inspect(synchronization.semaphores))

• Run with

make development
resty benchmark/synchronization.lua

• Before

semaphores: {
  foo = {
    name = "foo",
    sem = cdata<struct ngx_http_lua_sema_s *>: 0x7f0b18c48028,
    <metatable> = {
      __index = {
        count = <function 1>,
        new = <function 2>,
        post = <function 3>,
        version = "0.1.31",
        wait = <function 4>
      }
    }
  },
  <metatable> = {
    __index = <function 5>
  }
}

• After

semaphores: {
  <metatable> = {
    __index = <function 1>
  }
}