Faster joins: Avoid starting duplicate partial state syncs (#14844)

Currently, we will try to start a new partial state sync every time we
perform a remote join, which is undesirable if there is already one
running for a given room.

We intend to perform remote joins whenever additional local users wish
to join a partial state room, so let's ensure that we do not start more
than one concurrent partial state sync for any given room.

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There is a race condition where the homeserver leaves a room and later
rejoins while the partial state sync from the previous membership is
still running. There is no guarantee that the previous partial state
sync will process the latest join, so we restart it if needed.

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

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