innobase/include/srv0srv.h:
  Define SRV_CONCURRENCY_THRESHOLD
innobase/srv/srv0srv.c:
  Remove srv_thread_concurrency check from srv_conc_enter_innodb()
  and srv_conc_exit_innodb(), as the check is in the (only) caller
  of these functions, in ha_innodb.cc.
  srv_conc_force_enter_innodb(), srv_conc_force_exit_innodb():
  Check for srv_thread_concurrency >= SRV_CONCURRENCY_THRESHOLD
sql/ha_innodb.cc:
  Make use of SRV_CONCURRENCY_THRESHOLD