Fix for responding to a first request after socket activation
[Bug] When socket activated the first request might not be
properly handled because all the callback might not have
registered yet.
[Cause] N/A
[Solution] Delayed handling requests until all the callbacks have
been registered.
[Verification] Built, installed, run tests.
Did several tries with triggering socket activation:
1. systemctl start vasum.service
2. vasum-cli create_zone test
3. systemctl stop vasum.service
4. vasum-cli get_zone_ids
5. observe no error and list of zones should show 'test'
6. goto 3
Change-Id: I6de9742959d32afe68f496246065d3befc823955