Author: Alex Rousskov <rousskov@measurement-factory.com>

Author: Alex Rousskov <rousskov@measurement-factory.com>
Author: Amos Jeffries <squid3@treenet.co.nz>
API for subscribing AsyncCall handlers to event producers

This API allows AsyncCall handlers to be subscribed for receiving multiple
event callbacks from producer/factory classes.

Intended use-cases include the main port listeners which are started once.
Run for a long time. And over their lifetime need to generate multiple
calls to any one of several handlers without having specific type details
hard-coded about the calls they are spawning.
 ie they cannot use "new X(y,z)" because they are not aware of y and z.
    Nor can one AsyncCall be scheduled and fired multiple times.

Other use-cases already sighted are UDP readers and event timers.