Add sync_support for delayed delete mode (master)

[kanarip]

In sync_apply_unmailbox() and sync_apply_unuser(), the call to mboxlist_deletemailbox() is currently independent of whether delete_mode: delayed is configured, creating a risk of data-loss.

Under certain circumstances, in an A<->B two-way replication scenario for high availability, where B may have failed to synchronize a mailbox user/john to A, and A has a user/john mailbox created, the initial A->B "mailbox" replication is refused, but A will promote the synchronization to become of the "user" level, which B appears to accept; it'd remove the existing user/john mailbox, including the file-system hierarchy, resulting in (effectively) data-loss, before recreating the user/john mailbox.

From PR #4337, with @elliefm requesting the PR is rebased against master.

[kanarip]

I believe I could use some help;

In perl/imap/Cyrus/AccountSync.pm#L240, we would appear to call Cyrus::Mbname->new_intname("DELETED.user.newuser.63885DC8"), which appears to fail to set $self->{is_deleted}? But perl/imap/Cyrus/Mbname.pm#L29 appears to me should -- what am I missing?

[elliefm]

@kanarip Your most recent comment makes me think this is incomplete, but I don't know the answer to your question. The changes apply cleanly, but they don't pass our tests. Do you expect to keep working on this? If not I'll close it.