Move ServerConfigurationPayload to cluster.raft.persisted

Move ServerConfigurationPayload to cluster.raft.persisted

This introduces its mirror copy and modifies the old class
so that it readResolve()s to the new class. It also adjusts
all users to use the new class.

The new class uses Externalizable proxy pattern to allow the
class itself be evolved without breaking compatibility. Also
NoOpPayload is retrofitted this way, which makes all subclasses
of Payload not have their serialization format tied to Payload
itself.

Change-Id: I26010a9e1438dbc4cb1822e1c4dbb51e2b6e538e
Signed-off-by: Robert Varga <rovarga@cisco.com>