Don't fail /submit_token requests on incorrect session ID if request_token_inhibit_3pid_errors is turned on (#7991)

* Don't raise session_id errors on submit_token if request_token_inhibit_3pid_errors is set

* Changelog

* Also wait some time before responding to /requestToken

* Incorporate review

* Update synapse/storage/databases/main/registration.py

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

* Incorporate review

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

Add new method ratelimiter.can_requester_do_action and ensure that appservices are exempt from being ratelimited.

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>
Co-authored-by: Erik Johnston <erik@matrix.org>

It turns out that not all out-of-band membership events are labelled as such,
so we need to be more accepting here.

... otherwise it gets leaked.

It's just a thin wrapper around two ID gens to make `get_current_token`
and `get_next` return tuples. This can easily be replaced by calling the
appropriate methods on the underlying ID gens directly.

* Updated docs: Added note about missing 308 redirect support.

* Added changelog

The function is used for two purposes: 1) for subscribers of streams to
get a token they can use to get further updates with, and 2) for
replication to track position of the writers of the stream.

For streams with a single writer the two scenarios produce the same
result, however the situation becomes complicated for streams with
multiple writers. The current `MultiWriterIdGenerator` does not
correctly handle the first case (which is not an issue as its only used
for the `caches` stream which nothing subscribes to outside of
replication).

Closes: https://github.com/matrix-org/synapse/issues/6766

Equivalent Sydent PR: https://github.com/matrix-org/sydent/pull/309

I believe it's now time to remove the extra allowed `:` from `client_secret` parameters.

Turns out that part of the codebase (synapse.config.server) checks for this key explicitly. Remove that check.

Fixes https://github.com/matrix-org/synapse/issues/6583

Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>