Summary: Allow run-time deletion of counters and patterns of counters that will persist through istatd restarts.

Details:
* Add a delete.pattern command to AdminServer.
* * Most of the work will happen in the background, the command only triggers it.
* * * Ex:  echo -ne 'delete.pattern foo.bar.*\n' | bin/istatd_netcat -q 1 localhost 8112

* Deleting counters will now remove them from {{store}} on disk as well.
* * Will backup the tree of counters to {{store}}.bak with hardlinks.

* Keyslist for the front-end will be regenerated when counters are deleted.
* * The counters name will be removed from the counter map as well.

* New configuration knob.
* * prune-empty-directories-interval (120) - s - How often to prune empty directories from {{store}}.