Modify task counting in thread_queue.hpp

[msimberg]

Proposed Changes

• Make sure that new_tasks_count_ is decremented only after a corresponding count has been incremented (when moving tasks from one queue to another)
• This is to make sure new_tasks_count_ + thread_map_count_ never goes to 0 while there are tasks available (see below for example)

Any background context you want to provide?

This is relevant for suspending the runtime after launching a task

// in main thread

// no tasks in queues at this point
hpx::async(...); // task added to new_tasks
// scheduler on worker thread removes task from new_tasks (decrements count)
hpx::suspend(); // get_thread_count returns 0, runtime suspends even though
                // it should wait for the async task to run
// scheduler on worker thread adds task to thread_map (increments count)

I find this a bit ugly, but I expect it to be temporary. Once #3146 and a follow up PR to remove the new_tasks_ queue are merged, this will not be needed anymore.

A bit last minute, but this I would still like to have in the release assuming tests pass.

[hkaiser]

Excellent catch!