A graphite replication check

This check is most useful when you have a cluster of carbon-relays configured with REPLICATION_FACTOR > 1 and more than one graphite server those carbon-relays are configured to post to. This check ensures that replication is actually happening in a timely manner.

If this check returns OK you know your graphite data is successfully posting through the whole carbon-relay -> carbon-cache stack, even when you have many carbon-relays and many graphite servers with replication.