Updated single node lock manager to remove locks when they are no longer
in use.

USERGRID-1818
Updated to just use google cache.  Eviction will occur
automatically when locks not under heavy use are still in our cache.

USERGRID-1818
Updated comment
Merge pull request #178 from apigee/USERGRID-1818

Usergrid 1818
Merge branch 'master' into REST_Testing_Framework