Refactor PSK Credential retrieval interface

Refactor PSK Credential retrieval interface

When CA requests PSK credentials during DTLS handshake, SRM retrieves
credential for all devices and provides it to CA. This is un-necessary
and is in-efficient approach. Updated code to only provide credential
which has been requested by tinydtls.

Change-Id: Ie9bdc0ff1236b9f1f198215d211a682df416e731
Signed-off-by: Sachin Agrawal <sachin.agrawal@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/3009
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>