Send leader's full address via AppendEntries

Send leader's full address via AppendEntries

Added the leader's full actor address to AppendEntries. The
address can be rather long and we don't need to normally send
it so it's an Optional. The follower indicates via AppendEntriesReply
whether it needs the leader to send its address (new
needsLeaderAddress field). On receipt of the leader's address,
the follower sets it in its local RaftActorContext and notifies
the PeerAddressResolver. The ShardPeerAddressResolver impl
updates its local cache thus enabling transactions to resolve
the remote leader actor.

Since we're changing the serialized footprint of AppendEntries
and AppendEntriesReply, I preserved backwards compatibility by
versioning a new externalizable Proxy for each.

JIRA: CONTROLLER-1861
Change-Id: I1c0870a596b1782015eb973153b74dfcd48694e7
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>