dsync: Add hashed_headers setting This makes it possible to configure them

dsync: Use header hashing version 3

doveadm sync/backup: Added -S <max size> parameter to skip too large mails.

dsync: Don't export header hashes if remote mailbox is empty. This avoids doing a lot of work of reading through all mails' headers and hashing them, when the hashes aren't actually going to be used for anything.

dsync: Avoid finishing importer successfully on exporter failures. At least dsync_mailbox_import_changes_finish() could have been called when exporter was already failed.

dsync: When comparing headers' hashes to match messages, try to normalize the input. This is especially useful because some IMAP servers return different data depending on whether we're fetching only specific header fields, all headers or entire body. For now we're assuming that any non-ASCII is going to be replaced with '?', which helps at least with Zimbra and Yahoo. The header hashing algorithm is now versionable, so it can be modified more easily in future. This change should make imapc_features=zimbra-workarounds setting obsolete.

dsync: Use storage's mail_error to choose the doveadm exit code. Instead of always assuming that all errors are EX_TEMPFAIL.

dsync: Added -t <timestamp> parameter to save only mails newer than <timestamp> If one side has old mails that don't exist on the other side, they are ignored (not synced and not deleted).

dsync: Added DSYNC_BRAIN_FLAG_NO_MAIL_PREFETCH to avoid opening mails unnecessarily.

dsync: If verbose_proctitle=yes, show the current state in it.

dsync: Added support for syncing mailbox attributes.

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.