Add config and logic to allow all stat types to be deleted instead of sending zero or previous value

Similar to deleteCounters, this adds new config for:

deleteSets
deleteTimers
deleteGauges

and a control to turn them all on, with option to turn each off:

deleteIdleStats

Defaults to standard behavior of course.

this patches cleanly against master, as well as my other current PRs. 

Addresses the many concerns that have come up with sending zeros, including https://github.com/etsy/statsd/issues/95