Resource leak when tube is removed and -all is used

I was reading the source and thought I would report a tiny bug such that no one runs into it:

[`watchNewTubes()` method](https://github.com/99designs/cmdstalk/blob/fbd704da919d283643e673bd02efcf0221224b92/broker/broker_dispatcher.go#L82-L95) only starts new worker dispatchers, but never closes down preexisting ones if a tube is removed. If you have a lot of tubes being created and destroyed, you will run out of memory eventually.