cleanup deletion interface

 - add the delete method that works with instances of IdRev as well.
   This is more consistent with the interface that deals with saves,
 - no manifest is required when deleting a document, because we do not
   try to deserialize it, as it disappeared
[new] add document patch method

Users that want to work with document patches can now tell sohva to
patch a certain revision of a document and save the patched version into
the database.
If the revision to patch is not the last one, the standard automatic
conflict management process is started to try to solve the conflicts.