- added a new function to the database interface, cdb_close_cursor(). always

 - added a new function to the database interface, cdb_close_cursor().  always
call this when you're finished with a traversal but didn't bother reading all
the way to the end.

 - removed several cdb_begin_transaction()/cdb_end_transaction() calls that are
no longer needed because of the read-only cursor support.