HTTP/2: Support notification of closed/invalid connections.

[barchetta]

Provide an api such that application code can be notified when a connection is closed or becomes invalid. The use case is when a request comes in and the handler executes long-running business logic. If the client initiating the request drops the connection the customer would like to be notified so that they can terminate any long-running business logic.

This might only be possible with HTTP/2 because in that case the Helidon server has threads blocked in read on the socket and therefore can proactively detect dropped connections.