Refactor: Move ExecutorThread to its own file

Change-Id: I7e8d2049aac7fefa2289d9a4f171aafdb840d267
Reviewed-on: http://review.couchbase.org/31725
Reviewed-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>