Delete folder of metrics with: delcounters parent.path.*

Sorry this doesn't include tests. I manually confirmed that this works on counters, gauges, and timers.

Some Bonuses:
 * A minor refactor of the delete code in the management server. The five repeated lines for every delete command are now one.
 * The management server now warns you when you try to delete something, but it wasn't there yet.

I didn't add anything more complex like regex, because I didn't like the idea of having to escape the dots when issuing commands. This seems simple and to cover a lot of cases.