introduce Room.myMembership event
As you don't always have your own member with lazy loading
of members enabled, looking at the sync response section
where a room appears is the most reliable way of determining
the syncing user's membership in a room.
Before we already used this to read the
current room membership with `room.getMyMembership()`,
but we were still using the `RoomMember.membership` event
to detect when the syncing user's membership changed.
This event will help make those checks work well with LL enabled.
