This commit replaces the use of pthreads in the POSIX port with std::thread
and port::Mutex + port::CondVar. This is intended to simplify porting
the env to a different platform.

The indirect use of pthreads in PosixLogger is replaced with
std:🧵:id(), based on an approach prototyped by @cmumfordx@.

The pthreads dependency in CMakeFiles is not removed, because some C++
standard library implementations must be linked against pthreads for
std::thread use. Figuring out this dependency is left for future work.

Switching away from pthreads also fixes
https://github.com/google/leveldb/issues/381

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=212478311