Queue up initial leave requests to Matrix
Queue up initial leave requests to Matrix

Using an abomination of a QueuePool with "per-room" Queues, we can
get the desired behaviour of 1 in-flight req per room, max N rooms.

Fixes #300

matrix-org/matrix-appservice-irc,matrix-org/matrix-appservice-irc,matrix-org/matrix-appservice-irc