statsd plugin: Initial implementation.
statsd plugin: Add configuration for Host and Port.
statsd plugin: Implement the "Delete{Counters,Timers,Gauges}" options.
statsd plugin: Allow several metrics with the same name but different types.

This more closely mimicks Etsy's implementation.
stats plugin: Add support for sets.
statsd plugin: Implement the "TimerPercentile" configuration option.
fix the build
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.
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.
statsd plugin: Make metric names case-sensitive.

The StatsD by etsy is case sensitive, so we should do the same. Also, strcmp()
should have better performance than strcasecmp().
statsd plugin: Store values internally as double.

While the documentation states that "values should generally be integers", this
"should" is not taken in the strict RFC meaning. For compatibility with Etsy's
statsd, handle everything as a double up until the last possible moment.
sigrok: build system setup
stats plugin: Don't check the value of '(value_t).derive'.

statsd_parse_value() will write to (value_t).gauge, so don't read from another
member of the union. This also fixes problems with counter values being zero,
for whatever that is supposed to be useful for.

Github: #358
sigrok plugin
Use gcc-specific __attribute__ to mark unused parameter
Factor out device initialization
Config errors should not be fatal
Code cleanup