Verify all kernel messages are handled async and in order

[jasongrout]

Continuing from #3884, we should verify that all kernel messages are handled in order. Since at least some messages are handled async, all messages should be handled async. From the other issue:

In the classic notebook kernel, we took care that every kernel message handling function could return a promise all the way up the chain to the basic kernel message handling loop, and the kernel would not start handling the next message until the previous message was handled.

For example, the comm message handler should be able to pass back a promise, and kernel message handling doesn't proceed until that promise is fulfilled or rejected.