Vocabulary for message recipients

[kpreid]

There are three entities involved in receiving and processing a message, and we have clear technical terminology for only one of them. Find some better terms and update the documentation.

1. The type implementing the Listener trait; currently documented as “A Listener which…”. (This seems fine.)
2. The type which provides access to the results of the listener’s (1) work; currently document as “A Listener destination which …”, but I think that term needs improvement.
3. The task which reads and makes use of the data collected in the “destination” (2). I have no name at all for this; if anything, “destination” would be a better name for this than for (2).