Fix terminate sequence and observe option parser

Fix terminate sequence and observe option parser

1. Fix Observe option parser at stack
2. Fix terminate logic at stack
3. Add validation check to client
4. Remove JSONUtil class which multiply exist.
5. Rename Const file to Constants

Change-Id: Ib6419574151e787046b9a7fa89c351237304792a
Signed-off-by: Jee Hyeok Kim <jihyeok13.kim@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/6203
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Glen Youngjin Kim <glen.kim@samsung.com>