Rename Handler Nested & Collection

[gregw]

Fixes #8765 #8759 . Part of #9072

There is now a Handler interface hierarchy:

• Container is a Handler that has 1 or more contained Handlers.
• Wrapper is a Container with only 1 handler and a setHandler method.
• Collection is a Container with n handlers and a addHandler method

class are now:

• Abstract implements Handler
• AbstractContainer extends Abstract implements Container
• BaseWrapper extends AbstractContainer implements Wrapper
• Sequence extends AbstractContainer implements Collection

Lots of other associated cleanups

[gregw]

@sbordet @joakime @janbartel Can I get a review on this ASAP as I really don't want this to go stale and have to do it again.

[joakime]

I'm good with this.

[sbordet]

@gregw sorry I don't like some parts of this PR.

In every other Jetty class, Wrapper is a class and it is used to "wrap" an instance of an interface, so we should stick with this semantic.

BaseWrapper is not a good name.

I would rather have Wrapper back as a class, and if we really need it use Single as the interface opposed to Collection.

"Single" is also used in Java's Collections.singletonList() to return a list of 1 element ("Singleton" cannot be used as it remembers the well-known pattern which has a different meaning than what we want here).

[gregw]

@sbordet how about Delegate rather than Single?

[gregw]

Actually, I don't mind Singleton as it is now just as much associated with Collections.singletonXxx() than the pattern. Thus a Wrapper is a concrete implementation of a Singleton Container.

[basil]

Causes #12207, also decreasing the accuracy of the Javadoc a few lines above, which (now incorrectly) states:

Also put in a DefaultHandler so we get a nicer page than a 404 if we hit the root and the webapp's context isn't at root.