mdbox: Use mail_index_sync_set_reason() to give better reason messages for lock wait warnings.

mdbox: Check that m.X file doesn't have garbage at end of file when saving new data to it.

mdbox: Modified map locking behavior to avoid deadlocks when rebuilding storage. If both mailbox and map index need to be locked, the map index must now be locked first. Mailbox syncing optimistically tries to first sync without map locking, but if it sees expunges, it restarts with the map lock. The map lock is held now slightly longer during sync than before, but it shouldn't be noticeable. --HG-- branch : HEAD

mdbox: Renamed dbox_map* to mdbox_map*. --HG-- branch : HEAD

mdbox: Moving messages to alt storage is done done with doveadm altmove command. The command can take an arbitrary search query listing what messages should exist in alt storage. If the message has been copied to multiple mailboxes, the search query must match all the instances of the message. Since the search query can now be specified in command line, mdbox_altmove setting was also removed. --HG-- branch : HEAD

mdbox: Optimization: Don't update map UIDs' refcounts via array when it's not necessary. --HG-- branch : HEAD

mdbox: Removed forced automatic purging for now. It was too ineffecient. --HG-- branch : HEAD

Split dbox (single-dbox) and mdbox (multi-dbox) into separate storage backends. This cleans up the code, makes it faster and also fixes some bugs. Super-fast maildir migration code was also dropped, at least for now. --HG-- branch : HEAD