Fix deactivation running off the main process (#18716)

Best reviewed commit by commit.

With the new dedicated MAS API
(https://github.com/element-hq/synapse/pull/18520), it's possible that
deactivation starts off the main process, which was not possible because
of a few calls.

I basically looked at everything that the deactivation handler was
doing, reviewed whether it could run on workers or not, and find a
workaround when possible

---------

Co-authored-by: Eric Eastwood <erice@element.io>

docs/workers.md

@@ -238,6 +238,7 @@ information.
     ^/_matrix/client/unstable/im.nheko.summary/summary/.*$
     ^/_matrix/client/(r0|v3|unstable)/account/3pid$
     ^/_matrix/client/(r0|v3|unstable)/account/whoami$
+    ^/_matrix/client/(r0|v3|unstable)/account/deactivate$
     ^/_matrix/client/(r0|v3)/delete_devices$
     ^/_matrix/client/(api/v1|r0|v3|unstable)/devices(/|$)
     ^/_matrix/client/versions$