lib-storage: remove mdbox_purge_preserve_alt setting remove setting and its reference.

mdbox: Added mdbox_purge_preserve_alt setting to keep the file within alt storage during purge. Perhaps it would be better for this setting to be the default in v2.3 and maybe be removed entirely? Not a good idea to change during v2.2.x anyway, so for now this is a setting.

mdbox: Added mdbox_preallocate_space setting to preallocate size for newly created files.

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

mdbox: Removed quite pointless mdbox_purge_min_percentage setting. --HG-- branch : HEAD

mdbox: Removed unused mdbox_rotate_min_size setting. --HG-- branch : HEAD

mdbox_rotate_days changed to mdbox_rotate_interval, supporting one second resolution. --HG-- branch : HEAD

Added "size" setting type, which supports B/k/M/G/T suffixes. --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