References to the listeners are required by INodeBase, but these were
previously passed into the ctor from outside, rather than being owned and
controlled by this class. This, combined with the lack of support in ocean
for reopening a shutdown listener, made it impossible to implement a method
to restart the node's listeners.

(The ideal solution, of course, would be to add the missing restart support
to the select listener in ocean, but this will require a major swarm
release.)

(As a counterpart to the existing stopListener.)

There's more than one listener now.

This is intended as a replacement of the existing turtle.env.model.Node.
The reason for the new class is that the turtle/swarm node framework is
currently fairly complicated, with several different classes required to
implement a fake node. The new TestNode derives directly from NodeBase (the
standard base class for swarm nodes), whereas the older Node does not. This
means that to implement a fake node using TestNode, one does not have to
separately write a class deriving from NodeBase.