mdbox: Rebuild index after it's been fsck'd

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

mdbox: Added "mdbox_deleted" storage, which can be used to access messages with refcount=0 For example: doveadm import mdbox_deleted:~/mdbox "" mailbox inbox subject oops

mdbox: Flush/fsync newly saved mail data before locking map. This reduced the amount of time the map needs to be locked.

mdbox: If :INDEX=path is specified, keep storage indexes there also.

mdbox: Storage rebuild changes weren't actually being committed to disk. --HG-- branch : HEAD

mdbox: If one save fails, but others succeed, don't assert-crash at commit. --HG-- branch : HEAD

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

dbox, mdbox: Fixed race conditions when creating mailboxes. --HG-- branch : HEAD

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

mdbox: Avoid rebuilding storage if another process already did it. --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: Purging shouldn't give "unknown error" if mdbox storage doesn't exist. --HG-- branch : HEAD

lib-storage: Mailbox deletion API changed. Mailbox deletion should now be free of race conditions. The actual file deletion code is now responsibility of mailbox_list backend. --HG-- branch : HEAD

mdbox: Purging now also moves mails to alt storage (if it's used). mdbox_altmove setting specifies how old files should be moved. --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