graphite api: add support for sending metrics over UDP

There already exists an API for sending metrics to InfluxDB over TCP using
Graphite plaintext protocol. Sometimes it is more convenient to send metrics
over UDP, so adding that support for the Graphite as well. For now at least,
using the same port for UDP traffic than for TCP to make configuration really
simple.

UDP listener is disabled by default, but can be turned on by setting udp_enabled
to true in graphite plugin configuration.

There are already quite a few libraries and other solutions supporting Graphite UDP API, so this makes starting use of InfluxDB easier. One particular case where this is useful is when using StatsD as it natively supports using Graphite as the metrics backend, but it uses UDP to send metrics. This change should make InfluxDB a drop-in replacement for Graphite as StatsD backend.