Adds Cache-Control "stale-while-revalidate"

Adds Cache-Control "stale-while-revalidate" header parsing for better
server side cache entry expire setup.

see:
https://www.mnot.net/blog/2007/12/12/stale
http://tools.ietf.org/html/rfc5861
https://groups.google.com/a/chromium.org/forum/m/#!msg/chromium-dev/zchogDvIYrY/ZqWSdt3LJdMJ
Signed-off-by: Ralph Bergmann <ralph@the4thfloor.eu>

Change-Id: I1b7baf9997d3a8a251d21631a11deb503e3a7461
2 files changed