MINOR: threads: Avoid using a thread group mask when stopping.

MINOR: threads: Avoid using a thread group mask when stopping.

Remove the "stopped_tgroup_mask" variable, that indicated which thread
groups were stopping, and instead just use "stopped_tgroups", a counter
indicating how many thread groups are stopping. We want to remove all
thread group masks, so that we can increase the maximum number of thread
groups past 64.