add a lock to try to make this metric actually work

This should be safe to do on all workers/masters because it is guarded by
a config option which will ensure it is only actually done on the worker
assigned as a pusher.

It was originally implemented by pulling the full auth chain of all
state sets out of the database and doing set comparison. However, that
can take a lot work if the state and auth chains are large.

Instead, lets try and fetch the auth chains at the same time and
calculate the difference on the fly, allowing us to bail early if all
the auth chains converge. Assuming that the auth chains do converge more
often than not, this should improve performance. Hopefully.

Extends #5794 etc to the SimpleHttpClient so that it also applies to non-federation requests.

Fixes #7092.

Fixes #7043

This reverts commit 54dd2862, reversing
changes made to 6640460d.

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.

Move the default SAML2 error HTML to a dedicated file

Skip the correct visibility checks when checking the visibility of the state at a given event

Also add some JS to it to process any error we might have in the URI
(see #6893).

Co-Authored-By: Brendan Abolivier <github@brendanabolivier.com>

SAML2: render a comprehensible error page if something goes wrong