Better OnQueue handling.

Routes are queued and lazy-queued (via OnQueue::load()) more
consistently. For example OnQueue::load() loaded routes will take
priority of OnQueue::on() loaded routes if OnQueue::load()
is called before OnQueue::on() (in the same Closure). This didn't
work like this before. OnQueue::on() would always take priority
against OnQueue::load() regardless of calling order if in the same
Closure.

The code that handles this is also cleaner with less
stacks and queues (OnQueue::$_execution_stack doesn't exist any
more) and produce more reliable results.
Removed commented out code that isn't used any more.
Merge branch 'better-queue'