Simplify FakeVsysBackend shutdown handling.
Simplify FakeVsysBackend shutdown handling.

This change removes the FakeVsysBackend methods complete_shutdown_reader
and complete_take_too_long. Instead, all event handling is moved to
shutdown. And, because both commands ('take_too_long' and
'shutdown_reader') are mutually exclusive, a single event is used for
both cases. Docstrings are updated accordingly.

stephen-soltesz/collectd-mlab,m-lab/collectd-mlab,m-lab/collectd-mlab,stephen-soltesz/collectd-mlab