Release 4.2: DB->set_cache_priority

In previous releases, applications set the priority of a database's pages in the Berkeley DB buffer cache with the DB->set_cache_priority method. This method is no longer available. Applications wanting to set database page priorities in the buffer cache should use the mempset_priority() method instead. The new call takes the same arguments and behaves identically to the old call, except that a DB_MPOOLFILE buffer cache file handle is used instead of the DB database handle.