dsync: Add per-mailbox sync lock that is always used. Both importing and exporting gets the lock before they even sync the mailbox. The lock is kept until the import/export finishes. This guarantees that no matter how dsync is run, two dsyncs can't be working on the same mailbox at the same time. This lock is in addition to the optional per-user lock enabled by the -l parameter. If the -l parameter is used, the same lock timeout is used for the per-mailbox lock. Otherwise 30s timeout is used. This should help to avoid email duplication when replication is enabled for public namespaces, and maybe in some other rare situations as well.

dsync: Ignore missing remote mailbox when doing unidirectional sync If there are some folders on remote system that are being ignored by remote brain, do not error out.

dsync: Support syncing storages with 128bit GUIDs <-> string GUIDs.

dsync: Don't try to sync with GUIDs if we can't set them on the needed side. With two-way syncing both sides need to have writable GUIDs. With one-way syncing only the writing side needs to have writable GUIDs.

lib-storage: Mailbox attributes can now be accessed via istreams. The idea is to use istreams for larger values.

dsync: Added support for syncing mailbox attributes.

dsync: Automatically figure out which mailboxes can sync with message GUIDs.

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

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.