Change to improve dhcp-agent sync_state

Added a pool.waitall to dhcp-agent sync_state
to ensure that the last pool of threads are complete
before another sync_state begins.  Added another
log message to aid in debugging. Also added the
test_sync_state_waitall unit test.

Change-Id: I13d79a212672c4086e3082783743cb1ef2b48cfb
Closes-Bug: #1257514
Merge "Change to improve dhcp-agent sync_state"