Entity Ownership Service: need to exchange state for generations

The OpenFlow specification assumes strongly-consistent controllers in that each role change needs to specify the generation ID (a 64bit integer).

This means that nodes participating in control of a particular switch need to have a strongly-consistent view of what the current generation is.

The generation needs to change any time the set of candidates changes, or when a master/slave transition occurs.

Note that the fact this is 64 bit integer and its assignement rules are protocol-specific, hence the EOS APIs need to provide a way for OF to plug in and manage this piece of state.