Bug fix: Call ticker.Stop() when aggregator is stopped

[byronwolfman]

TL;DR

This is a bugfix PR that prevents the aggregator's ticker from being leaked when the statsd client is closed.

Longer version

For reasons that aren't super interesting, we have an app that periodically allocates and closes statsd clients, e.g.

d, err := statsd.New("")
// Do some stuff
err = d.Close()

The app does this frequently enough that we discovered a leaked ticker. The DataDog continuous profiler helped zero in on the problem by hinting that the app was spending an increasing number of CPU cycles on runtime.CheckTimers and runtime.netpoll/runtime.epollwait. For a one-liner change, the effect is pretty outsized! You can spot the difference:

[scottopell]

Thanks for fixing this and writing up a detailed PR, this change looks great.