When enabling the account validity feature, Synapse will look at startup for registered account without an expiration date, and will set one equals to 'now + validity_period' for them. On large servers, it can mean that a large number of users will have the same expiration date, which means that they will all be sent a renewal email at the same time, which isn't ideal.
In order to mitigate this, this PR allows server admins to define a 'max_delta' so that the expiration date is a random value in the [now + validity_period ; now + validity_period + max_delta] range. This allows renewal emails to be progressively sent over a configured period instead of being sent all in one big batch.

If account validity is enabled in the server's configuration, this job will run at startup as a background job and will stick an expiration date to any registered account missing one.

Merge pull request #5187 from matrix-org/dbkr/only_check_threepid_not_in_use_if_actually_registering

Only check 3pids not in use when registering

Allow client event serialization to be async

Expose DataStore._get_events as get_events_as_list

Use correct config option for ratelimiting in tests

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

We checked that 3pids were not already in use before we checked if
we were going to return the account previously registered in the
same UI auth session, in which case the 3pids will definitely
be in use.

https://github.com/vector-im/riot-web/issues/9586

This is in preparation for reaction work which requires it.

Migrate all tests to use the dict-based config format instead of hanging items off HomeserverConfig (#5171)