Improve replication metrics

We are trying to report on replication lag and find that there is no good single metric to do that.
ageOfLastShippedOp is close, but unfortunately it is increased even when there is nothing to ship on a particular RegionServer.

I would like discuss a few options here:
Add a new metric: replicationQueueTime (or something) with the above meaning. I.e. if we have something to ship we set the age of that last shipped edit, if we fail we increment that last time (just like we do now). But if there is nothing to replicate we set it to current time (and hence that metric is reported to close to 0).
Alternatively we could change the meaning of ageOfLastShippedOp to mean to do that. That might lead to surprises, but the current behavior is clearly weird when there is nothing to replicate.

Comments? jdcryans, stack.

If approach sounds good, I'll make a patch for all branches.

Edit: Also adds a new shippedKBs metric to track the amount of data that is shipped via replication.