When figuring out which topological token to start a purge job at, we
need to do the following:

1. Figure out a timestamp before which events will be purged
2. Select the first stream ordering after that timestamp
3. Select info about the first event after that stream ordering
4. Build a topological token from that info

In some situations (e.g. quiet rooms with a short max_lifetime), there
might not be an event after the stream ordering at step 3, therefore we
abort the purge with the error `No event found`. To mitigate that, this
patch fetches the first event _before_ the stream ordering, instead of
after.

This means that things like to device messages don't get blocked behind
processing PDUs, which can potentially take *ages*.

This is pretty pointless. Let's just use SynapseError.

Fix a bug where we would assign a numeric userid if somebody tried registering
with an empty username

Updates to the SAML mapping provider API

AdditionalResource really doesn't add any value, and it gets in the way for
resources which want to support child resources or the like. So, if the
resource object already implements the IResource interface, don't bother
wrapping it.

Fixes #6402

... for consistency with other modules, and because we'll need it sooner or
later and it will be a pain to introduce later.

Refuse to start if sqlite is older than 3.11.0

this saves doing it on each connection, and will allow us to pass extra options
in.

We might not need the cursor at all.

Signed-off-by: Manuel Stahl <manuel.stahl@awesome-technologies.de>

Fix media repo admin APIs when using a media worker.

Fix typo in message retention policies doc

Fixes #6552 

This was ill-advised. We can't modify verify_keys here, because the response
object has already been signed by the requested key.

Furthermore, it's somewhat unnecessary because existing versions of Synapse
(which get upset that the notary key isn't present in verify_keys) will fall
back to a direct fetch via `/key/v2/server`.

Also: more tests for fetching keys via perspectives: it would be nice if we actually tested when our fetcher can't talk to our notary impl.

Add complete documentation of the message retention policies support