Makes statsd optional

[ChrisLovering]

In #42 we tried to make statsd optional while adding it, however the way we attempted to do so made it so that editors wouldn't detect that the async statsd client inherited methods from stats client base.

[wookie184]

If we want to make statsd an optional dependency we shouldn't expose StatsClientBase directly (or by inheriting from it). Instead we should create a wrapper class (e.g. an ABC), with one implementation that uses statsd, and have a separate implementation (that doesn't do anything) for when statsd is not installed.