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.


include/hash.h:
  - remove current_record from HASH, instead modify hash_first,
  hash_next to accept HASH_SEARCH_STATE as an IN/OUT parameter
mysys/hash.c:
  - remove HASH::current_record
  - change declarations of functions that use HASH in read-only mode
    to accept const HASH * instead of HASH *. 
  - implement hash_search; move the old implementation of hash_search
  to hash_first
mysys/testhash.c:
  - adjust the test case to changed function declarations
sql/lock.cc:
  - adjust to changed declarations of hash_search, hash_next
sql/sql_acl.cc:
  - adjust to changed declarations of hash_search, hash_next
sql/sql_base.cc:
  - adjust to changed declarations of hash_search, hash_nex
sql/sql_cache.cc:
  - adjust to a changed declaration of hash_replace