BUG/MEDIUM: threads: ignore max-threads-per-group when thread-groups is set

BUG/MEDIUM: threads: ignore max-threads-per-group when thread-groups is set

As documented, max-threads-per-group is the default number of threads
to arrange in a group before creating another group, and is only meant
to be used when thread-groups is not set.

However it was always enforced, so configs like:

   global
       thready-groups 2

which were sufficient in 3.2 and above to start with 64-128 threads
are now suddenly limited to 32 threads! Let's relax the limit when
thread-groups is set!

No backport is needed since this is only 3.4.