Make innodb_thread_concurrency 0 by default, and extend the usable
 range from 0 to 1000 (0 disables the thread throttling).


innobase/include/srv0srv.h:
  Remove SRV_CONCURRENCY_THRESHOLD. The magic value is 0 from now on.
innobase/srv/srv0srv.c:
  Remove SRV_CONCURRENCY_THRESHOLD. The magic value is 0 from now on.
mysql-test/r/innodb.result:
  innodb_thread_concurrency is 0..1000 now, and the default is 0.
mysql-test/t/innodb.test:
  innodb_thread_concurrency is 0..1000 now.
sql/ha_innodb.cc:
  Remove SRV_CONCURRENCY_THRESHOLD. The magic value is 0 from now on.
sql/mysqld.cc:
  innodb_thread_concurrency is 0..1000 now, and 0 (the default)
   disables the thread throttling.