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
(cherry picked from commit 9f6ff7e312e6d6ddfc520bcfa8a4f40cf5806845)
Merge "Change to improve dhcp-agent sync_state" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable