Compress Digest and MutableDigest

For large clusters, STABLE messages are quite large, and should be compressed to be sent over the wire.

STABLE messages are sent between members in the same view, so we could only send the ViewId + highest_delivered/highest_received seqnos.

Everybody who receives a STABLE message grabs the View associated with the ViewId (should be the current view !) and creates a Digest based on the View and the long[] array.

Further optimization:

• Canonicalize digests: if everyone has (14) 20, 22, then we could write it once, give it an ID of (say) 1 and then only refer to 1 again if we encounter the same digest. Actually, as a matter of fact, most of the digests would be the same, so this optimization could have a big effect !