* the has_key() method was not raising a DBError when a database error
   had occurred. [SF patch id 1212590]
 * added a wrapper for the DBEnv.set_lg_regionmax method [SF patch id 1212590]
 * DBKeyEmptyError now derives from KeyError just like DBNotFoundError.
 * internally everywhere DB_NOTFOUND was checked for has been updated
   to also check for DB_KEYEMPTY.  This fixes the semantics of a couple
   operations on recno and queue databases to be more intuitive and results
   in less unexpected DBKeyEmptyError exceptions being raised.