ytrafficserver plugin to refresh content asynchronously while serving stale data

http://www.mnot.net/drafts/draft-nottingham-http-stale-while-revalidate-00.txt 

Version 0.0.17
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* max_age config param added. 

Version 0.0.16
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* Pass on connecting IP to HttpConnect
* use INKHttpIsInternalRequest to check if request is local or not

Version 0.0.15
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* handle max stale age correctly for only-if-cached + background-fetch

Version 0.0.14
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* SWR overrides only-if-cached
* Set cache lookup status to FRESH if within SWR window even for only-if-cached

Version 0.0.13
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* set cache lookup status to MISS after doing SWR for only-if-cached

Version 0.0.12
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* Fix for bug [3341089]
* Support for background fetch on cache miss

Version 0.0.11
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* Fix for bug [3338245]

Version 0.0.10
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* use pristine URL in SWR sub request

Version 0.0.9
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* Respect must-revalidate

Version 0.0.8
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* Fix for bugs [3260033]

Version 0.0.7
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* Fix for bugs [3260033 3282115 3275888]

Version 0.0.6
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* Fix for bug [3263080]

Version 0.0.5
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* Fix for bug [3262768]

Version 0.0.4
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* Set cache status after doing SWR

Version 0.0.3
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* No more time-to-wait support

Version 0.0.2
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* Use INKHttpTxnCacheRespGet to get SWR information

Version 0.0.1
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* initial version