lwt: do not go over the replica list in case a quorum is already reached
lwt: do not go over the replica list in case a quorum is already reached

Also add a comment that clarifies why doing prune before learning on all
replicas is safe.

Message-Id: <37dd2c4c7296a0d23255f9e95d0fc92e17b4f2c5@scylladb.com>

avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla