statsD: "set" datatype, "delete_idlestats" Option, minor tweaks

* adds "set" datatype for statsD
* adds "delete_idlestats" option to remove no-longer-received values (with some sub options)
* special gauge handling with "delete_idlestats"
* added "count_ps" metric for timers
* send zero values for empty timers if not "delete_idlestats" (to immitate etsy/statsD behavior)