Mv hot threads1

This is branch contains the threading code originally developed for the eleveldb async NIF.  It was previously delivered in leveldb as part of a patch in branch 1.2.4-turner.  Joe B. spotted an unlikely, but real, race condition that this branch addressed via the QueueThread object.

Today only Imm to Level-0 compactions post to a hot thread pool.  The goal is to move background disk writes and all background compactions to a series of specialized hot thread pools ... but that will be a different branch.