[Dovecot] Using dsync+imapc for migration from other IMAP servers to Dovecot

Dovecot v2.1 hg (http://hg.dovecot.org/dovecot-2.1) now supports doing e.g.:

dsync -o plugin/imapc_user=remote.user@example.com -o plugin/pass=remote-password mirror imapc:imap.example.com

dsync will preserve message UIDs and UIDVALIDITY/UIDNEXT values for mailboxes, just like with all other storage backends.

A couple of bad things:

1. It currently fetches one message at a time from server. Very slow.. Will be fixed some day.

2. You can't do incremental sync, because dsync relies on message GUIDs, which aren't available in IMAP protocol. This could be fixed some day. Also a support for Dovecot extension could be added some day to make this work for Dovecot as remote IMAP server.