Improved handling of in-progress consumer buffer rotation and simplified peek of mpmc xadd q

[franz1981]

This PR include an alternative fix for #255 on a0bc103#diff-61853935c955f7d3dd424601f34d4ca1R583:
a pooled chunk element is considered "stable" right after its sequence is being written, so it can be read after winning the cas.

There are other improvements/fixes in this PR:

1. an in-progress rotation doesn't prevent anymore racing consumers to return null from poll if q is empty: on a0bc103#diff-61853935c955f7d3dd424601f34d4ca1L528
2. rotateConsumerBuffer is responsible to await next to be visible after winning the cas (on a0bc103#diff-61853935c955f7d3dd424601f34d4ca1R443) with no fear to get interruped by other consumers (on a0bc103#diff-61853935c955f7d3dd424601f34d4ca1L612)
3. poll contains more comments to explain the covered cases
4. peek has been simplified

NOTE: #265 could be solved by incorporating some of the changes on this PR

[coveralls]

Pull Request Test Coverage Report for Build 570

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 47.581%

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Totals
Change from base Build 551:	0.0%
Covered Lines:	1672
Relevant Lines:	3514

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💛 - Coveralls

[franz1981]

@nitsanw Now is ready to be reviewed

[nitsanw]

@franz1981 I'm going to merge the PRs for the Xadd queues are review the implementations from scratch before the release.

[franz1981]

@nitsanw Bud, let me know if you need to chat about it (hangout or on github/gitter) to make it fast 👍