Summary: util/threadpool.cc's function name is the same as a well-known class name. It breaks unity build. Rename it.
Test Plan: Run all existing test.
Reviewers: yiwu, IslamAbdelRahman
Reviewed By: IslamAbdelRahman
Subscribers: leveldb, andrewkr, dhruba
Differential Revision: https://reviews.facebook.net/D58881
Conditionally retrofit thread_posix for use with std::thread
and reuse the same logic. Posix users continue using Posix interfaces.
Enable XPRESS compression in test runs.
Fix master introduced signed/unsigned mismatch.