Change websocket cleanup performance from O(n^2) to O(n).

[marshallbrekka]

Move subscription tracking to a dict keyed by the underlying websocket object.

Changes the cleanup from two nested for loops (the 2nd being the unsubscribe_from method), to a single loop with (the common case) deleting a single key from a dict.

We found that with a sufficient number of disconnected clients, and long gaps between database activity, the cleanup could end up taking multiple hours. This would cause the python web service to become unresponsive.

[savingoyal]

lgtm! we can ship this once @saikonen approves

[marshallbrekka]

@savingoyal @saikonen is there any update here, anything I can do to help get this upstreamed? thanks!