actor increments
the reference count by one. So if three threads were simultaneously making
upcalls on a handler, the reference count would be four.</li>

<li>
Assuming an external event (and thread) decides to close the handler. It
simply removes the handler from the Reactor. This decreases the reference
count to three.</li>

<li>
As each thread completes their upcall, the reference reduces.</li>

<li>
Once the final thread exits the upcall, the reference reaches zero, and
the handler is finally deleted.</