sdbox: Commit expunges to index first before unlinking the files.

single-dbox: Fixed broken mailbox handling. --HG-- branch : HEAD

single-dbox: Simplified syncing code. --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