Use Mutex instead of Monitor

[ganmacs]

Signed-off-by: Yuta Iwama [email protected]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:

I used Mutex when no need reentrant lock since Mutex is faster than Montior.

require 'benchmark/ips'

mo = Monitor.new
mu = Mutex.new

Benchmark.ips do |x|
  x.report("monitor") do
    mo.synchronize {}
  end

  x.report("mutex") do
    mu.synchronize {}
  end
end

Warming up --------------------------------------
             monitor   224.461k i/100ms
               mutex   371.210k i/100ms
Calculating -------------------------------------
             monitor      3.987M (± 2.4%) i/s -     19.977M in   5.013755s
               mutex     10.221M (± 2.5%) i/s -     51.227M in   5.015317s

Docs Changes:

no need

Release Note:

same as title

[repeatedly]

Looks good.