Fix race condition in dispatcher

[jtlisi]

Fixes: #2182

By ensuring operations on the cancel and ctx field are wrapped with a mutex this removes the race condition from the dispatcher.

[simonpasquier]

LGTM. Would you mind adding the test that you wrote in the original issue?

[jtlisi]

@simonpasquier I added the test from the original issue. Still not passing the CI test cases but the failures look unrelated to this PR.

[simonpasquier]

Thanks! The CI is a bit flaky sometimes. I've restarted it and will merge on green.

[simonpasquier]

Ah the failure is legit! Alertmanager crashes on startup because the Stop() function is called with a nil receiver. We need this:

       if d == nil {
           return
       }
	d.mtx.Lock()
	if d.cancel == nil {
            d.mtx.Unlock()
	    return nil
	}
        ...

[simonpasquier]

Thanks!