Moved the router processing queue code to the agent/common
directory and renamed it "resource processing queue". This
way it can be consumed by other agents, or possibly even
moved to neutron-lib in the future.
Conflicts:
neutron/agent/l3/agent.py
neutron/tests/unit/agent/l3/test_agent.py
Change-Id: I735cf5b0a915828c420c3316b78a48f6d54035e6
(cherry picked from commit f24f3b6b7b)