Added support for proactive link checking

Added support for proactive link checking

In current implementation whenever user tries to add link between two devices
which already exists apis logs error and doesn't return callback.
In this change, link api first checks that link between two intended devices
exists or not, if it exists then api return error and doesn't move ahead.

Change-Id: Iac02a44542235b962de2fe965f45548d96f9c625
Signed-off-by: Randeep Singh <randeep.s@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2899
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Sachin Agrawal <sachin.agrawal@intel.com>