have multiple threads executing
upcalls on a handler that needs to be shutdown in a safe manner. The following
illustrates an example of how this mechanism works:
<ul>
<li>
When a handler is created, it reference count is one.</li>

<li>
After the handler is registered with the Reactor, it reference count becomes
two.</li>

<li>
At this point, the handler creator can let go of the handler reference,
bringing down the reference count to one.</li>

<li>
For each thread executing upcalls on the handler, the Re