Summary:
this is the ground work for separating memtable flush jobs to their own thread pool.
Both SetBackgroundThreads and Schedule take a third parameter Priority to indicate which thread pool they are working on. The names LOW and HIGH are just identifiers for two different thread pools, and does not indicate real difference in 'priority'. We can set number of threads in the pools independently.
The thread pool implementation is refactored.

Test Plan: make check

Reviewers: dhruba, emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12885