ALLJOYN-1691: Add a delay before stopping timerthreads.

Timerthreads will idle for WORKER_IDLE_TIMEOUT_MS time
before stopping.

Change-Id: I5bef2532965222137885ad74cf002d0aba44bcde
ALLJOYN-1691: Added missing call to Lock().

Change-Id: I1fac31cd545e889f8655a6588832723536ffcf07