Eliminate accumulated forwarded Request messages

[sergefdrv]

Using messagelog to buffer forwarded requests eliminates dandling goroutines when the destination replica stops receiving the messages. However, this will still accumulate the forwarded messages in the unicast message logs, whereas we only need to keep at most one request per client (until we add support for concurrent client requests).
This is because each new Request may be added to the unicast message log for forwarding but never removed from there.

Originally posted by @sergefdrv in #166