[IOT-1939] Fix for failure of test cases in C++ Notification Service due to time...

[IOT-1939] Fix for failure of test cases in  C++ Notification Service due to time delay

Unit test of notification service sometimes fails due to time delay.
In this patch, network related procedure was separated in C++ layer of notification service.
The changes are based on the C layer changes from the patch "https://gerrit.iotivity.org/gerrit/#/c/17397"

Change-Id: Ib0c859f9052be2049135226cc6abd9f314875b09
Signed-off-by: Poovizhi <poovizhi.a@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/18197
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Uze Choi <uzchoi@samsung.com>