statsd plugin: Remove the prefix in the counter and gauge handlers.

The prefix is added in statsd_metric_lookup_unsafe(), so we don't need
to do this here anymore. Since snprintf() is expensive, this increases
performance considerably.
statsd plugin: Avoid calling snprintf() in the hot path.

snprintf() is very expensive and the alternative is simple. This shaves
off about 7% of the time spent handling events.