Modify AddCredential API to prevent invalid CredID generation

Modify AddCredential API to prevent invalid CredID generation

Three patches combined together
1. Add checking credential uniqueness
- Prevent credential duplication by checking when saving credential
http://suprem.sec.samsung.net/gerrit/#/c/92486/
2. Update the CompareCredential API
- Update CompareCredential API to improve readability
http://suprem.sec.samsung.net/gerrit/#/c/93190/
2. Modify AddCredential API to prevent invalid CredID generation
- Prevent invalid CredID generation
http://suprem.sec.samsung.net/gerrit/#/c/93042/

Patch #3-4: Build error fixed
Patch #5: Old comment removed

Change-Id: I14c16ade352c2011d6b124c374a6b1cece1a0d4d
Signed-off-by: Jongmin Choi <jminl.choi@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/12407
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Greg Zaverucha <gregz@microsoft.com>
Reviewed-by: Randeep Singh <randeep.s@samsung.com>