Shard Client Reader for registration

[neilisfragile]

No description provided.

[erikjohnston]

Next steps: We think that we can already do this now that sessions are persisted to the database, but we should test and double check this is true. Ideally we'd also add some unit tests to assert that registration works if you switch workers half way through

[richvdh]

to be clear: we already have multiple client readers; this task is to remove the rules in haproxy that pin registration requests to a single instance.

[clokep]

I believe that #7530 is related to this, although that's specific to using SAML.

[clokep]

@erikjohnston had a couple more thoughts on this:

FYI on the client reader sharding the first step might be to look at the federation sharding test and steal the logic for running multiple workers, and then do a registration flow against them but change which worker you speak to each time