Guard against spurious wakeups in devicediscoveryserver.cpp

Guard against spurious wakeups in devicediscoveryserver.cpp

Like most condition variable implementations, std::condition_variable may
wake up spuriously, even though it is not signalled (see
http://en.cppreference.com/w/cpp/thread/condition_variable/wait).  Since
the intention of blocking on the std::condition_variable in main is to
wait forever, add a loop to block again after spurious wakeups.

Change-Id: I97e89a1ec95d6ba65eeb7daff861ee0a65d03a7e
Signed-off-by: William Dieter <william.r.dieter@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/357
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Sudarshan Prasad <sudarshan.prasad@intel.com>