Improved Observe: can't observe replication under mutation

In case there wasn't a jira tracking issue for this already...

Attempting to observe the replication of a newly created item is possible, where the client can ask the replicas. However, trying to observe replication of a mutating item is not possible with the current protocol, where the client cannot detect whether its mutation was replicated successfully (versus other concurrent client mutating the same item, versus the replication failing).

A protocol change with new commands that also return sequence number (in addition to the existing CAS number) could fix this.