Big Switch: Lock consistency table for REST calls

Adds a new class to hold an SQL table lock throughout
a REST call to prevent servers in an HA deployment from
using a stale consistency hash value.

Also passes the current context down to the server manager
for every request since the consistency hash will need to
be read from the database instead of from a variable on the
server pool object.

Conflicts:

	neutron/plugins/bigswitch/plugin.py
	neutron/tests/unit/bigswitch/test_servermanager.py

Closes-Bug: #1328331
Change-Id: I5f8402c076d7732742c0ae4d9b9d6833d42a0b7b
(cherry picked from commit 09e706b210513968cac2bad78fe81ee9cbb5b5be)
Merge "Big Switch: Lock consistency table for REST calls" into stable/icehouse