Addresses #770

• Add an option for the callback on receiver to be blocking or nonblocking
• Updated blocking_test to ensure when pollRoutines is 1 AND it's blocking, the events are processed in serialized manner.

Manual Testing

Run the example receiver code: https://gist.github.com/liu-cong/a132fe8629715419c7a58ee20001d071
It retrieves the Ce-Sleep attribute from the event and sleep for that period of time.

Then send a few events to the receiver using curl,

curl -v localhost:8080
  -X POST \
  -H "Ce-Sleep: 20s" \
  -H "Ce-Id: 1" \
  -H "Ce-Specversion: 1.0" \
  -H "Ce-Type: type" \
  -H "Ce-Source: source" \
  -H "Content-Type: application/json" \
  -d '{"msg":"Test!"}

Without blockingCallback flag

cloudevents.NewClient(p, client.WithPollGoroutines(1))

Sent 3 events, finish at the same time

With blockingCallback flag

cloudevents.NewClient(p, client.WithBlockingCallback(), client.WithPollGoroutines(1))

Finished one after another

Signed-off-by: James Yu jyu@confluent.io