dsync: Include messages_count in the mailbox states. This allows detecting that stateful dsync can't be done when message count is suddenly wrong.

dsync: Merge last-common-* values from both local and remote when they differ.

dsync: Use private modseqs to support syncing private flags in shared mailboxes.

doveadm sync/backup: Added -s <state> parameter to do a fast stateful sync. Initially use an empty string for the input state. The output state is written to stdout.

Initial version of dsync rewrite. * doveadm backup not implemented at all yet * syncing mailbox renames is somewhat broken (at least renaming \noselect mailboxes) * saving/restoring "state" is implemented by dsync brain, but not by doveadm. this should be easy to do, just need to figure out how the replication code wants it.