- Fix new caching issue when broken/misleading object data was stored in database.
  The object must be cleaned up before storing in DB, because some code
  (out of rcube_message and rcube_imap classes) is changing objects data
  e.g. parts mimetype or body. Now it's become a problem because we're
  saving the object in cache at shutdown (after all modifications).
- Make stored message object smaller by removing some redundant data.