fix: Use sync.WaitGroup to start eventsoures and sensors

[whynowy]

Use sync.WaitGroup to start the eventsource service and sensor service, this brings the ability to let the goroutines finish cleaning up when a stop signal issues.

Disabled update-mocks.sh in codegen, since the mock files are not used currently.

[zachaller]

Now I get what expected as well, thanks

{"level":"info","ts":1596719845.3426712,"logger":"argo-events.eventsource","caller":"eventsources/eventing.go:336","msg":"Shutting down...","eventSourceName":"kafka"}
{"level":"info","ts":1596719845.3427703,"logger":"argo-events.eventsource","caller":"eventsources/eventing.go:278","msg":"exiting eventbus connection daemon...","eventSourceName":"kafka"}
{"level":"info","ts":1596719845.342825,"logger":"argo-events.eventsource","caller":"kafka/start.go:151","msg":"terminating: context cancelled","eventSourceName":"kafka","eventSourceType":"kafka","eventName":"example"}
{"level":"info","ts":1596719845.3853364,"logger":"argo-events.eventsource","caller":"kafka/start.go:140","msg":"Error from context: context canceled","eventSourceName":"kafka","eventSourceType":"kafka","eventName":"example"}