Properly implement login rate limiting

[DRSchlaubi]

Currently we connect all shards at once and rate limit the identify command, which causes session resets and timeouts (see #624)

A better solution would be to implement rate limiting for logins

[lukellmann]

You mean taking into account session_start_limit (most importantly max_concurrency) like described in Sharding, right?

[DRSchlaubi]

Yes