I realized after merging the previous PR that it is difficult to
correctly pass ownership into a timer, because something like:
TimerID x = omq.add_timer([&] { omq.cancel_timer(x); }, 5ms);
doesn't work when the timer job needs to outlive the caller. My next
approach was:
auto x = std::make_shared<TimerID>();
*x = omq.add_timer([&omq, x] { omq.cancel_timer(*x); }, 5ms);
but this has two problems: first, TimerID wasn't default constructible,
and second, there is no guarantee that the assignment to *x happens
before (and is visible to) the access for the cancellation.
This commit fixes both issues: TimerID is now default constructible, and
an overload is added that takes the lvalue reference to the TimerID to
set rather than returning it (and guarantees that it will be set before
the timer is created).
Updates `add_timer` to return a new opaque TimerID object that can later
be passed to `cancel_timer` to cancel an existing timer.
Also adds timer tests, which was omitted (except for one in the tagged
threads section), along with a new test for timer deletion.
