statsd plugin: Dont'a call common.c:parse_value().

parse_value() is doing a strdup() do it can remove trailing whitespace.
We don't need that for the statsd protocol, so we just call strtoull()
instead, improving performance but a couple of percent again.
statsd plugin: Don't use strtok_r() to split multi-metric packets.

Profiling has shown that it is one of the bottle-necks.