Refactor the way group calls hang up (#3234)
* Refactor how group call end calls
We previously used disposeCall to terminate the call which meant that
sometimes a call would never get a hangup event. This changes it so
that we always end a call by calling hangup, then do the cleanup
when the hangup event arrives, so the cleanup is the same whether
we hang up or the other side does.
* Some fixes for failing & hanging tests
* Add type for the call map
