when high concurrency": remove HASH::current_record and make it
an external search parameter, so that it can not be the cause of a 
race condition under high concurrent load.
The bug was in a race condition in table_hash_search,
when column_priv_hash.current_record was overwritten simultaneously
by multiple threads, causing the search for a suitable grant record
to fail.
No test case as the bug is repeatable only under concurrent load.