Use Concurrent.available_processor_count to set default thread pool max threads #277

Job	Run time
test-sample-app	19s
rubocop	23s
test (2.5, rails-5.2.gemfile)	38s
test (2.5, rails-6.0.gemfile)	38s
test (2.5, rails-6.1.gemfile)	37s
test (2.6, rails-5.2.gemfile)	45s
test (2.6, rails-6.0.gemfile)	42s
test (2.6, rails-6.1.gemfile)	42s
test (2.7, rails-5.2.gemfile)	43s
test (2.7, rails-6.0.gemfile)	43s
test (2.7, rails-6.1.gemfile)	42s
test (2.7, rails-7.0.gemfile)	41s
test (3.0, rails-6.0.gemfile)	25s
test (3.0, rails-6.1.gemfile)	21s
test (3.0, rails-7.0.gemfile)	22s
test (3.1, rails-6.0.gemfile)	24s
test (3.1, rails-6.1.gemfile)	26s
test (3.1, rails-7.0.gemfile)	24s
test (3.1, rails-main.gemfile)	45s
test (3.2, rails-6.0.gemfile)	23s
test (3.2, rails-6.1.gemfile)	23s
test (3.2, rails-7.0.gemfile)	22s
test (3.2, rails-main.gemfile)	29s
test (3.3, rails-6.0.gemfile)	25s
test (3.3, rails-6.1.gemfile)	21s
test (3.3, rails-7.0.gemfile)	22s
test (3.3, rails-main.gemfile)	28s
test (jruby-9.2, rails-5.2.gemfile)	1m 0s
test (jruby-9.2, rails-6.0.gemfile)	6h 0m 15s
test (jruby-9.2, rails-6.1.gemfile)	6h 0m 12s
test (jruby-9.3, rails-5.2.gemfile)	6h 0m 13s
test (jruby-9.3, rails-6.0.gemfile)	6h 0m 12s
test (jruby-9.3, rails-6.1.gemfile)	50s
test (jruby-9.4, rails-6.0.gemfile)	6h 0m 13s
test (jruby-9.4, rails-6.1.gemfile)	6h 0m 14s
test (jruby-9.4, rails-7.0.gemfile)	6h 0m 15s
test (jruby-9.4, rails-main.gemfile)	6h 0m 15s
	2d 0h 17m 32s

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