Add unique IP statistic
Add unique IP statistic

This statistic keeps track of how many unique IPs veneur has received
packets from. We can't report this to datadog/dogstatsd in the read or
parsing loop (additional IO in those code paths is expensive enough to
cause packet drops), so we offload the metric to a worker instead.

evanj/veneur,gphat/veneur,stripe/veneur,gphat/veneur,stripe/veneur,evanj/veneur,gphat/veneur,evanj/veneur