Add wait_for_tasks()

[DeveloperPaul123]

This PR builds on top of #61 and adds a wait_for_tasks() functions.
Fixes #23

[GabTux]

I've tested and reviewed the code, and it functions flawlessly. I compiled the test suite with both Clang and GCC, using thread and address sanitizers. All test runs produced zero errors.

The only thing I am concerned about is performance. Consider a scenario where all threads are awake, spinning in the inner loop (while (pending_tasks_.load(std::memory_order_acquire) > 0);). The queues are empty, but one thread is finishing a long-running task, so the pending_tasks_ remains non-zero. The threads would iterate through the queues to get a task and rotate their IDs in the priority queue.

I benchmarked the current master version and this pull request using the first two count primes benchmarks. The results were very similar. So I think it is acceptable because that case does not happen so often.

[DeveloperPaul123]

@GabTux Awesome thanks for testing things out.

Nice catch with the case of one long running task keeping the rest of the threads awake. Maybe we do need 2 counters after all; maybe with just different names.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.24%. Comparing base (cee2fce) to head (8fe6151).
Report is 17 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   98.07%   98.24%   +0.16%     
==========================================
  Files           2        2              
  Lines         104      114      +10     
==========================================
+ Hits          102      112      +10     
  Misses          2        2              

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