Jetty 9.4.x 4331 async close complete

[gregw]

Fix for #4331 Implement async close and/or complete when PENDING or UNREADY.
This includes PR #4377

[gregw]

@sbordet, @lachlan-roberts. I would not mind a little bit of review as I work through this PR... to make sure I don't commit too much effort in a wrong direction.
So far I have:

• merged in the pending PR Issue #4376 Async Content Complete #4377 (would be good to get that one merged)
• added test cases for Improve handling of HttpOutput.close() for pending writes #4331 to AsyncCompletionTest
• begun a major refactor of HttpOutput so that it's state is kept in HttpChannelState rather than separately. This will eventually allow us to deal with completion and close under a single atomic lock rather than having two locks. So far I like how it has simplified many write methods, but error handling is a mess and I'm dubious about some of locations that I callback into the state... for example onWriteComplete feels a little wrong... but it is difficult because a single API write may result in many actual writes.... In Jetty-11 we are going to have to split HttpChannelState into a core protocol state and an API state.

[gregw]

Closing this as a dead end. After review with @sbordet it was felt that moving HttpOutput.State into HttpChannelState was a wrong direction an that the HttpOutput should know it's own API state. However there were some good cleanups, so we need to extract some ideas into a new branch and PR.