Only run prepare_database on connection for in-memory databases.

Fixes #6569.

Fixes #6575

Previously we tried to be clever and filter out some unnecessary event
IDs to keep the auth chain small, but that had some annoying
interactions with state res v2 so we stop doing that for now.

 Use the filtered version of an event when responding to /context requests for that event

Merge branch 'babolivier/fix-context-filter' of github.com:matrix-org/synapse into babolivier/fix-context-filter

Co-Authored-By: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

This fixes a weird bug where, if you were determined enough, you could end up with a rejected event forming part of the state at a backwards-extremity. Authing that backwards extrem would then lead to us trying to pull the rejected event from the db (with allow_rejected=False), which would fail with a 404.

The main point here is to make sure that the state returned by _get_state_in_room has been authed before we try to use it as state in the room.

When we perform state resolution, check that all of the events involved are in
the right room.

When we do an event auth operation, check that all of the events involved are
in the right room.

When we request the state/auth_events to populate a backwards extremity (on
backfill or in the case of missing events in a transaction push), we should
check that the returned events are in the right room rather than blindly using
them in the room state or auth chain.

Given that _get_events_from_store_or_dest takes a room_id, it seems clear that
it should be sanity-checking the room_id of the requested events, so let's do
it there.

Make it return the state *after* the requested event, rather than the one
before it. This is a bit easier and requires fewer calls to
get_events_from_store_or_dest.

This is a non-functional refactor as a precursor to some other work.

There was a bunch of unnecessary conversion back and forth between dict and
list going on here. We can simplify a bunch of the code.

Sometimes the filtering function can return a pruned version of an event (on top of either the event itself or an empty list), if it thinks the user should be able to see that there's an event there but not the content of that event. Therefore, the previous logic of 'if filtered is empty then we can use the event we retrieved from the database' is flawed, and we should use the event returned by the filtering function.

Give the server config to the RoomWorkerStore

Make the PusherSlaveStore inherit from the slave RoomStore

So that it has access to the get_retention_policy_for_room function which is required by filter_events_for_client.