Add waitgroup for listeners

[joe94]

During a code review (#119 (comment)), @kristinaspring suggested using a waitgroup for our listeners. I think adding this will be helpful as a way to prevent unexpected behavior in listeners doing long running final updates after Stop() has finished.

argus/chrysom/client.go

Line 279 in dd35ffe

c.ticker.Stop()

[joe94]

I'm seeing this as a good opportunity to make the listeners restartable as well. That is, we should be able to make the sequence of operations Start(), Stop(), Start() without any issues or resource leaks.