Improve dovecot directory layout for performance

[feld]

This is going to bite us sooner or later. Our directory scans for many operations that happen without us knowing are going to get very expensive as soon as relays start accumulating hundreds of thousands of accounts. We can reduce the pain with a two-level sharded directory structure which is also often used by object stores (S3 clones) to achieve the same result.

Dovecot actually supports this. One example would be:

mail_location = maildir:/home/vmail/mail/DOMAIN/%1Mu/%2.1Mu/%u

We would also need to solve this in Filtermail so it knows how to calculate where the account directory is meant to be.