BUG#47073 - valgrind errs, corruption,failed repair of partition,
            low myisam_sort_buffer_size

Fixed race conditions discovered with the provided test case and
stabilized test case.

include/myisam.h:
  Serialize submission of messages from multi-threaded REPAIR.
mysql-test/r/myisam.result:
  REPAIR output highly depend on threads activity. Disabled
  result log to make test case deterministic.
mysql-test/t/myisam.test:
  REPAIR output highly depend on threads activity. Disabled
  result log to make test case deterministic.
storage/myisam/ha_myisam.cc:
  Serialize submission of messages from multi-threaded REPAIR.
storage/myisam/mi_check.c:
  Serialize submission of messages from multi-threaded REPAIR.
storage/myisam/sort.c:
  Only master thread is allowed to detach write cache from
  the share.