The NS_NewTimer* family of functions, when using a custom event target,
currently go through a path that looks something like:
auto timer = createTimer()
timer->SetTarget(target);
// call the requisite Init* function
return timer;
This setup is inefficient, because SetTarget requires the timer mutex to
be acquired. The mutex acquisition here is completely unnecessary,
because the timer hasn't yet been shared out to the wider world; we can
set the timer target without acquiring the mutex at all because we know
that no sharing is possible at this point.
This patch reworks things somewhat to make that possible.
6d7cf47871