Periodically clean the dead server list

dead servers appear to only be cleaned up when a server comes up on the same host and port; however, if HBase is running on smth like YARN with many more hosts than RSes, RS may come up on a different server and the dead one will never be cleaned.
The metric should be improved to account for that... it will potentially require configuring master with expected number of region servers, so that the metric could be output based on that.
Dead server list should also be expired based on timestamp in such cases.