Fixes#7065
This is basically the same as https://github.com/matrix-org/synapse/pull/6847 except it tries to populate events from `state_events` rather than `current_state_events`, since the latter might have been cleared from the state of some rooms too early, leaving them with a `NULL` room version.
Currently we rely on `current_state_events` to figure out what rooms a
user was in and their last membership event in there. However, if the
server leaves the room then the table may be cleaned up and that
information is lost. So lets add a table that separately holds that
information.
