Change ReplicatedLogImplEntry to Externalizable proxy pattern

Change ReplicatedLogImplEntry to Externalizable proxy pattern

The Externalizable proxy pattern decreases serialized sized as
it avoids serializing the full class info. We need to keep
ReplicatedLogImplEntry for persistence backwards compatibility.
A new SimpleReplicatedLogEntry class was added to implement the
Externalizable proxy pattern. ReplicatedLogImplEntry "readResolves"
to SimpleReplicatedLogEntry. Also, SimpleReplicatedLogEntry
implements MigratedSerializable to cause a snapshot on upgrade
enabling us to remove ReplicatedLogImplEntry in the next release.

Change-Id: Iaf5c8f6b271c3ead43a80ac905412b4cb5a9efa2
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>