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

doveadm-sync: Add end-date support

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

dsync: Add support for features Add empty_header_workaround as first feature

lib-mail, dsync: Moved dsync_mail_hash_more() to lib-mail/message-header-hash.*

dsync: Improved header hash v2 algorithm to remove repeated '?' chars. This is to help with Yahoo that replaces UTF-8 chars in headers with a single '?' (instead of '?' per each 8bit byte).

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: 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: saved-date doesn't need to be looked up until mail body is being read. This should improve the performance when the saved-date isn't already cached or otherwise quickly accessible. This change also makes dsync slightly incompatible with earlier versions. When using dsync with an earlier version the saved-dates aren't synced. It would be too much trouble to try to preserve full backwards compatibility, especially because saved-date doesn't matter so much and isn't even visible to IMAP clients.

dsync: Give hints to lib-storage about what fields we're going to fetch.

dsync: Don't crash when requested mail doesn't have a GUID.

dsync: Various importer fixes. Handle unexpectedly wrong mailbox state better.

dsync: Fixed syncing message keywords. "add" and "final" can't be mixed together always without causing keywords to be added back unintentionally.

dsync: When syncing locally, mailbox_copy() the mail to attempt a fast copy.

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.