Make innodb_thread_concurrency 0 by default, and extend the useable
range from 0 to 1000 (0 disables the thread throttling).
Remove SRV_CONCURRENCY_THRESHOLD (20). The magic value to
disable the thread throttling is 0 from now on.
Work around Bug #12071: Do not call os_file_create_tmpfile() at runtime.
Instead, create all tempfiles at startup and guard access to them with
mutexes.