MEDIUM: tasks: check the global task mask instead of the thread number

MEDIUM: tasks: check the global task mask instead of the thread number

When deciding whether to scan the global run queue or not, we currently
check the configured threads number, and if it's 1 we skip the queue
since it's not supposed to be used. However when running with a master
process and multiple threads in the workers, the master will turn this
number back to 1 while some task wakeups might possibly have set bits
in the global tasks mask, thus causing active_tasks_mask to have one
bit permanently set, preventing the process from sleeping.

Instead of checking global.nbthread, let's check for the current
thread's bit in global_tasks_mask. First it will make this part of the
code more consistent, working like a test and set operation, it will
solve the issue with master+nbthread and as a bonus it will save a
lock/unlock for each scheduler call when the thread doesn't have a
task in the global run queue.