Add raftVersion field to AppendEntriesReply

Add raftVersion field to AppendEntriesReply

Added a raftVersion field to AppendEntriesReply and modified
AbstractLeader to store the raftVersion in the FollowerLogInformation.
This will enable sending the appropriate serialized version for
subsequent messages sent to the follower.

The raftVersion in the FollowerLogInformation is initialized to
0 (HELIUM_VERSION) so we assume the oldest version for backwards
compatibility until we get the first AppendEntriesReply with the
follower's actual version.

Since we're adding a new field to AppendEntriesReply this won't break
backwards compatibility as Java serialization handles that. The
raftVersion will be set to 0 if sent from a pre-boron version.

Change-Id: I4519c4f314674840f2578848b2888c3e8467dd21
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>