Make use of thread pool for sending metrics async

[56quarters]

Use the Rust thread pool library for writing metrics to the buffer asynchronously. The thread pool size would be user controlled and passed to the sink that makes use of it. Each call to MetricSink.emit would submit a task to the thread pool.

Questions:

• Should this be a new sink or should this be part of the BufferedUdpMetricSink from Investigate buffered MetricSink implementation #18?
• Should there be some background thread that flushes the buffer no matter what every N milliseconds?

[56quarters]

It's looking like this will just be an AsyncMetricSink that writes metrics to another sink in a thread. It will have no knowledge of buffering or whatnot and just delegate to another sink. It might make sense to go back and change BufferedUdpMetricSink to just do buffering, not have direct knowledge that it is writing to a UDP socket.

[56quarters]

Given that Sender and ThreadPool can't be shared between threads, the async metric sink is going to require a slightly different use case. We should create some examples (in docs and code) showing how you could safely use both the async version and plain buffered version.