Add support for multiple timers in one packet

I need statsdpy to support relaying from hosts which are collecting
metrics statsd-style internally. Counters can just be incremented by
more than 1. Gauges are fundamentally incompatible with having multiple
setters. That just leaves timers.

This solution is simple and minimally invasive. All I do is make it so
that I iterate over "fields" (the stuff after the first colon, which is
|-separated) and treat each one as a timer value.