Faster joins: Fix incompatibility with restricted joins (#14882)

* Avoid clearing out forward extremities when doing a second remote join

When joining a restricted room where the local homeserver does not have
a user able to issue invites, we perform a second remote join. We want
to avoid clearing out forward extremities in this case because the
forward extremities we have are up to date and clearing out forward
extremities creates a window in which the room can get bricked if
Synapse crashes.

Signed-off-by: Sean Quah <seanq@matrix.org>

* Do a full join when doing a second remote join into a full state room

We cannot persist a partial state join event into a joined full state
room, so we perform a full state join for such rooms instead. As a
future optimization, we could always perform a partial state join and
compute or retrieve the full state ourselves if necessary.

Signed-off-by: Sean Quah <seanq@matrix.org>

* Add lock around partial state flag for rooms

Signed-off-by: Sean Quah <seanq@matrix.org>

* Preserve partial state info when doing a second partial state join

Signed-off-by: Sean Quah <seanq@matrix.org>

* Add newsfile

* Add a TODO(faster_joins) marker

Signed-off-by: Sean Quah <seanq@matrix.org>

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