can be used directly as the super-class of single-threaded daemons.  There is
currently no need for an intermediary SingleThreadedDaemon class.

tape bridge and vdqm daemons will eventaully inherit from one of these new
classes.  This commit is a step towards this goal.  Please note that this
commit modifies the castor::rh::Server and castor::server::BaseDaemon classes
in not so desirable ways.  The castor::rh::Server has been modified so that it
does not use the MetricCollector class.  The castor::server::BaseDaemon class
has been modified so that it does not use either the UDPListenerThreadPool or
the MetricCollector class.  I will put back this code in another commit.