dbox: Reverted the "check if altpath is mounted" change for now. If alt path didn't yet exist, it reported bogus errors. It should somehow check if there is anything under the alt root dir.

dbox: If alt path is lost, fail with an error rather than rebuilding indexes. This could be simply because alt storage is unmounted at the time and rebuilding would just lose messages. If this error happens unintentionally, it's still possible to fix it with "doveadm force-resync".

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: Removed mdbox_max_open_files setting. It's now hard coded to 2, and the files are closed after 30 seconds of being unused. --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

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