Thread Observer
Summary: Observer methods, so users of IOThreadPoolExecutor can do stuff when threads are added/removed. As a use case, previously the thrift server only used the threads already started when it started up, and assumed iothreadpool was never resized.
Test Plan: Added several unittests
Reviewed By: jsedgwick@fb.com
Subscribers: trunkagent, doug, fugalh, alandau, bmatheny, mshneer, folly-diffs@
FB internal diff:
D1753861
Signature: t1:
1753861:
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54cbdfee0efb3b97dea35faba29c134f2b10a480