Refs #9059

I want to create listeners to check the buffersEvents() method, but
it's undesirable to have this make the network connection.
N.B. I couldn't use a default argument because that led to either
a warning about overriding a method, or an ambiguity error.

Deviating from what's currently written in the design document because:
(a) I don't think that maintaining listener lifetime here and handing
    out references to existing listeners will work out well - e.g. if
    two MonitorLiveData instances were created they would 'compete'
    in calling ILiveListener::extractData.
(b) Even if we did there's no need for a separate LiveListenerManager
    class (see UnitFactory which does the same thing).