Improve doveadm mailbox delete speed

[jrse]

Currently the mailbox delete process follows the delete process implemented by mdbox.
This involves deleting mailbox by mailbox in a sequencial way.
When deleting thousands of mailboxes at the same time this process may take a long time.

It is up for discussion how this can be improved.
There are currently several ideas how to improve this.

1. Delete all mails in the users namespace (this requires separate namespace for each user, which is the default.) (may be slower than 2)
2. read all user index mailbox files, and delete each mail with the guid asynchronously.