Some boost versions have a conflicting overload of wait_until that returns void.
Explicitly use a template here to avoid hitting that overload.
Github-Pull: #6285
Rebased-From: 72bf90d770ce5b2653fd482928646cd6a9f5f6d7
Don't clear `stopRequested` and `stopWhenEmpty` at the top of
`serviceQueue`, as this results in a race condition: on systems under
heavy load, some of the threads only get scheduled on the CPU when the
other threads have already finished their work. This causes the flags to
be cleared post-hoc and thus those threads to wait forever.
The potential drawback of this change is that the scheduler cannot be
restarted after being stopped (an explicit reset would be needed), but
we don't use this functionality anyway.
On a busy or slow system, the CScheduler unit test could fail because it
assumed all threads would be done after a couple of milliseconds.
Replace the hard-coded sleep with CScheduler stop() method that
will cleanly exit the servicing threads when all tasks are completely
finished.
jl777