This attempts to be a direct port of https://github.com/matrix-org/synapse-dinsic/pull/74 to mainline. There was some fiddling required to deal with the changes that have been made to mainline since (mainly dealing with the split of `RegistrationWorkerStore` from `RegistrationStore`, and the changes made to `self.make_request` in test code).
The `remote_media_cache_thumbnails_media_origin_media_id_thumbna_key`
constraint is superceded by
`remote_media_repository_thumbn_media_origin_id_width_height_met` (which adds
`thumbnail_method` to the unique key).
PR #7124 made an attempt to remove the old constraint, but got the name wrong,
so it didn't work. Here we update the bg update and rerun it.
Fixes#8649.
It landed in schema version 58 after 59 had been created, causing some
servers to not run it. The main effect of was that not all rooms had
their chain cover calculated correctly. After the BG updates complete
the chain covers will get fixed when a new state event in the affected
rooms is received.
