* Fixes#8993 - Retainability of special Chunks
* Restored Jetty 11's AsyncContentProducer and related classes in jetty-ee9-nested module (src and test).
* Introduced Retainable.canRetain().
* Removed Chunk.isTerminal() and replaced it with alternative method calls.
* Clarified AsyncContent.write() in case of Chunk.Error.
* Removed AsyncContent.write(Chunk, Callback) because it was making the API confusing.
For example, AsyncContent.close() clashing with write(EOF, NOOP), or
AsyncContent.fail(x) clashing with write(Chunk.Error, NOOP), etc.
* Improved usage of Chunk.from(..., Retainable).
* Improved usage of Chunk.slice().
* Using from() in MultiPart, rather than duplicating code.
* Fixed MultiPart.Parser.Listener.onPartContent() javadocs.
* Renamed non-retaining Chunk.from() to Chunk.asChunk().
* Removed Chunk.slice() methods, inlining them where necessary.
* Carefully reviewed all usages of read()-like methods that return a Retainable instance to make sure it is released.
* Updated HTTP/2 and HTTP/3 usages of Stream.Data to follow the correct retain/release semantic.
Signed-off-by: Simone Bordet <simone.bordet@gmail.com>
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