Added lock logic in findResource callback for C++ samples

Added lock logic in findResource callback for C++ samples

As part of Gerrit review #101,(83fed5a6c6b01d32de3e47c669652a1aa79ebb49)
there were changes required in other samples to prevent a crash in the
findResource callback.

Additionally, made minor correction in groupclient by changing
printf to cout in one of the function

Change-Id: I524f5d862c82011e5d008e686cc1d6e41ac3f170
Signed-Off-by:Sudarshan Prasad<sudarshan.prasad@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/126
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Erich Keane <erich.keane@intel.com>
Reviewed-by: Sashi Penta <sashi.kumar.penta@intel.com>