sql/ha_innodb.cc:
  Remove unused variable innobase_thread_concurrency.
  Add UNIV_LIKELY hints to srv_thread_concurrency tests
  and lower the limit from 500 to 20.
sql/ha_innodb.h:
  Remove unused variable innobase_thread_concurrency.
sql/mysqld.cc:
  Make innodb_thread_concurrency=20 by default (it was 8).