Faster joins: Update room stats and the user directory on workers when finishing join (#14874)

* Faster joins: Update room stats and user directory on workers when done

When finishing a partial state join to a room, we update the current
state of the room without persisting additional events. Workers receive
notice of the current state update over replication, but neglect to wake
the room stats and user directory updaters, which then get incidentally
triggered the next time an event is persisted or an unrelated event
persister sends out a stream position update.

We wake the room stats and user directory updaters at the appropriate
time in this commit.

Part of #12814 and #12815.

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

* fixup comment

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