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

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