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>