[0.94] ageOfLastShippedOp replication metric doesn't update if the slave regionserver is stalled

In replicationmanager
HRegionInterface rrs = getRS();
rrs.replicateLogEntries(Arrays.copyOf(this.entriesArray, currentNbEntries));
....
this.metrics.setAgeOfLastShippedOp(
this.entriesArray[currentNbEntries-1].getKey().getWriteTime());
break;

which makes sense, but is wrong. The problem is that rrs.replicateLogEntries will block for a very long time if the slave server is suspended or unavailable but not down.

However this is easy to fix. We just need to call refreshAgeOfLastShippedOp();
on a regular basis, in a different thread. I've attached a patch which fixed this for cdh4. I can make one for trunk and the like as well if you need me to do but it's a small change.