Use /members api for lazy loading
This commit is a substantial change, as /members returns state events,
not profile information as /joined_members, and this allows to simplify
the implementation quite a bit. We can assume again all members have
a state event associated with it.
I also changed most of the naming of lazy loaded members to
out-of-band members to reflect that this is the relevant bit for most
of the code, that the members didn't come through /sync but through
another channel.
This commit also addresses the race condition between /(joined_)members
and /sync. /members returns the members at the point in the timeline
at a given event id. Members are loaded at the last event
in the live timeline, and all members that come in from sync
in the mean time are marked as superseding the out of band members,
so they won't be overwritten, even if the timeline is reset in the
mean time.
Members are also marked if they originate from an out-of-band channel
(/members) so they can be stored accordingly (future PR).
The loading status is kept in room state now, as this made resolving
the race condition easier. One consequence is that the status needs
to be shared across cloned instances of RoomState. When resetting
the timeline (and cloning the room state) while lazy loading is in
progress, one of the RoomStates could be left in progress indefinitely.
Though that is more for clarity than avoiding any actual bugs.
