Statsd host updating + async send

[haljin]

Kicking off this PR so we can have some more discussion on this hopefully. :)

This PR adds two things:

1. Ability to change the statsd hostname and/or port on the fly
2. Asynchronous sending of metrics

I'm not super happy with the solution for 2. however I'd like to hear your opinions first whether I should even go that way. What I'd like to do further is to:

• Add a pool of metric-sending worker processes,
• Dispatch each metric to a random member of that pool, instead of all routing through the single TelemetryMetricsStatsd process
• Replace the ETS pool for the async_send: true case with the pool as it does not make sense in that context
• Clean up some duplicate code

[haljin]

Closing due to comments on #46 Replaced by #48