Fixes #308: Remove useless fences on chunk's prev field accesses

[franz1981]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 682

• 4 of 6 (66.67%) changed or added relevant lines in 2 files are covered.
• 29 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.5%) to 85.763%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
jctools-core/src/main/java/org/jctools/queues/MpUnboundedXaddArrayQueue.java	2	3	66.67%
jctools-core/src/main/java/org/jctools/queues/MpUnboundedXaddChunk.java	2	3	66.67%

Files with Coverage Reduction	New Missed Lines	%
jctools-core/src/main/java/org/jctools/maps/NonBlockingHashMapLong.java	3	79.15%
jctools-core/src/main/java/org/jctools/maps/NonBlockingSetInt.java	3	77.65%
jctools-core/src/main/java/org/jctools/maps/NonBlockingHashMap.java	5	79.04%
jctools-core/src/main/java/org/jctools/maps/NonBlockingIdentityHashMap.java	7	76.06%
jctools-core/src/main/java/org/jctools/maps/ConcurrentAutoTable.java	11	47.69%

Totals
Change from base Build 681:	-0.5%
Covered Lines:	4753
Relevant Lines:	5542

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💛 - Coveralls