Entity Ownership Service: support graceful state handoff

Current entity ownership model does not support stateful failovers, as is the case in changing ownership of a OF connection.

Essentially there are multiple resources attached to a particular entity: the TCP session which dominates the ability to be a candidate and a datastore transaction chain which is attached to the master role.

When a master is becoming the slave as part of election, it needs to clean up data store state before the new master can resume operations. This effectively means that if there is a master and the election results indicate that the role needs to be moved to another candidate, we need a 'stopping' transition, which is asynchronous and completed by the outgoing master. Once the old master has stopped the new master can be elected.

In the OF case, this would mean that outgoing leader would initiate TransactionChain.close() and finish 'stopping' when the transaction chain callback indicates the shutdown procedure has completed.