after merge.

Concurrent read and update of privilege structures (like simultaneous
run of SHOW GRANTS and ADD USER) could result in server crash.

Ensure that proper locking of ACL structures is done.

No test case is provided because this bug can't be reproduced
deterministically.


sql/sql_acl.cc:
  Ensure that access to ACL data is protected by acl_cache->lock mutex.
  Use system_charset_info for host names consistently.
  Remove check_acl_user().  Use find_acl_user() instead.
sql/sql_acl.h:
  Remove check_acl_user() declaration.
sql/sql_parse.cc:
  Use is_acl_user() instead of check_acl_user().