We want to be able to mock out the upcoming `Room.release()` method.

We use the implementation of the RELEASE operation provided by the room
lifecycle manager, and implement the spec points relating to room map
bookkeeping and releasing the underlying realtime channels.

Based on [1] at 6f0740a.

I have not implemented the spec points that relate to making sure that a
room fetch waits for any previous room with the same ID to finish
releasing; this is a part of the spec which is in flux (currently
implemented via the INITIALIZING status, which was added to the spec
after we started implementing the room lifecycle manager and hasn’t been
implemented in this SDK yet, and soon to be further changed in the spec
by making room-getting async). We can look at the current state of
things when we come to do #66.

Part of #47.

[1] ably/specification#200