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)
Ensure that session is rolled back on bulk creates

During bulk creates, the session is began explicitely;
ensure that it gets rolled back before re-raising in
order to avoid triggering InvalidRequestError
exceptions when the session is reused.

Partial-bug: 1244757

Change-Id: I797faeec2ca5374620db905599ab6e6f04fdcabd
(cherry picked from commit cb3f86dd781587eaa9fca6cc4a2f98de93526db7)
Updated from global requirements

Change-Id: Icedcd7285cd13d9b4e480e9ee4800f22d3804c7c
Merge "Change to improve dhcp-agent sync_state" into stable/havana
Merge "Ensure that session is rolled back on bulk creates" into stable/havana