Always persist ServerConfigurationPayload log entries

Always persist ServerConfigurationPayload log entries

We need to always persist ServerConfigurationPayload log entries
regardless of whether or not persistence is enabled for the derived
RaftActor's data.

I added a new tagging interface PersistentPayload, implemented by
ServerConfigurationPayload, to indicate a Payload
needs to always be persisted. Since log entries are persisted by
both the RaftActor and Follower behavior via the ReplicatedLog, the
logic to determine persistence based on PersistentPayload needs to be
available to both. The ReplicatedLog uses the persistence provider
contained in the RaftActorContext which is the
DelegatingPersistentDataProvider set by the RaftActor. So to keep
the rest of the code the same and keep it simple, I derived a
RaftActorDelegatingPersistentDataProvider which overrides persist to
handle the PersistentPayload logic utilizing the RaftActor's
existing PersistentDataProvider.

Change-Id: I243026b28ed57461ad92324b6947091ae74a7127
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>