xadd qs producer buffer rotation can be improved

[franz1981]

Both the xadd qs are able to recycle chunks and save allocations while offering.
During a producer buffer rotation while appending a pooled
chunk there is a small window where offering threads see a NIL chunk as producerBuffer, preventing them to backward navigate the list of chunks to reach the target one. It cannot happen with a non-pooled chunk.

The appending algorithm requires that producerBuffer must always contain the buffer with the highest index or NIL (ie index := -1) in order to allow offering threads to reach their target chunk to write the element into or just wait until a valid chunk is available. The publishing of chunks happens by calling soProducerBuffer, that need to be called exclusively to preserve the ordering guarantee above: the exclusivity is ensured by casProducerChunkIndex. A stalled producer could see a pooled chunk turning its chunk index into the last one right before entering the casProducerChunkIndex, making it able to succeed. It means that
https://github.com/JCTools/JCTools/blob/master/jctools-core/src/main/java/org/jctools/queues/MpscUnboundedXaddArrayQueue.java#L317
is the last instruction that would be executed exclusively by a single thread and soProducerBuffer has to be called before it to ensure the ordering.

This issue could be solved similarly to #265 marking the rotation state (using a parity bit or -2) and making racing producers able to make progress or just "wait" before attempting to append a new chunk.
This would allow to publish producerBuffer only when is fully initialized.

Just a note: newChunk.soIndex(nextChunkIndex) need to remain an ordered store, because stalled producers need to see a stable/correct newChunk::prev ie soIndex will write release prev to them.