Move throttle code to independent source file.  Clean out some dead code within env_posix.cc
SetWriteRate() really should be using the gThrottleMutex.  Added that.
move more functional throttle code to the throttle.cc file.  Not quite a class yet.  But cleaner.
Merge pull request #86 from basho/mv-riak-1-4-cleanup

Mv riak 1 4 cleanup