Moved the module RequestQueue down to Common.
Moved ExplorationMode and Purity down to Parallel.

This was done because they might be used
by people not writing back-ends, and my
goal has been to have Common function as
a form of attic that is only needed by
people writing a back-end.
Now the Process ignores exceptions thrown while acknowledging WorkerQuit.

Once WorkerQuit has been received we know that the supervisor doesn't
actually need to hear from us any more, so if the connection is closed
before we get a chance to reply acknowledging receipt of WorkerQuit then
this is not only not a big deal but in fact is expected of some adapters
(such as network on Linux).