Add txStatsD module

txStatsD is Canonical's port of Etsy's StatsD to Python. We currently use
Etsy's StatsD, but I'd like to evaluate txStatsD as a replacement.

What I like about txStatsD is that it uses the Twisted framework to get all the
scaffolding of a polling network daemon, including a flexible command-line
runner and a simple plug-in architecture. We can use the plug-in architecture
to implement simple regex-based metric filters that let you specify which
summary statistics should be computed and to which backend they should be
routed. This is in contrast with Etsy's StatsD, which doesn't provide hooks for
this sort of thing.

txStatsD also provides a Python API for implementing metric-reporting clients.
There is the potential there for making our various metric modules
singificantly simpler and shorter. For an example client, see:
<https://github.com/sidnei/txstatsd/blob/master/example-stats-client.tac>

I plan to remove the statsd module if txStatsD proves to be a suitable
replacement.

Change-Id: I19ecf608d64a9ed13fbd3dfa41bd28b43f5a7bd1