Introduces a version manager in the callbacks rpc, which tracks
the agents' known resource versions across the distributed system,
making rpc callback push notifications smart about which resource
versions need to be serialized and sent over the wire during a
cloud upgrade process.
Subsequent patches will implement the callback to fetch agent
resource versions from database, the status updates with
agent resource versions included, and the fast path agent to
neutron-servers fanout call with version details as defined
in change I02b694137eb2d58e5f2f3e7631f0e4b90f7c17ad
Related-Bug: #1535247
Change-Id: I3fb49ae6fe237a926225b508bc8f0286426bf532
cb18881770