Rudy/batch metrics

Allow to send multiple metrics in a single udp packet.
To send them in batch, use `BatchedDatadogstatsD` instead of `DatadogStatsd` and add a call to `BatchedDatadogstD::flush_buffer()` at the end.

To avoid to create too large UDP packets, if more than 50 metrics are reported, it will automatically be flushed.

Similar to https://github.com/DataDog/dogstatsd-ruby/pull/7 and https://github.com/DataDog/dogstatsd-python/pull/19