Fix for responding to a first request after socket activation

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