UI, history: update using xhr date header

[carlfeberhard]

Makes the 'updated since' contents query based on any previous update's
ajax Date response header instead of the client time. This may offer
more protection against incorrect client time.

This works (and may work better than the previous method) because the
Date header is floored and not rounded and seems to mark a time before
the contents query is made.

Resolves #2242 (hopefully)

[nsoranzo]

@carlfeberhard I think this should target release_16.04, if possible.

[mvdbeek]

Definitely fixes #2242 for me, thanks for working this out @carlfeberhard !
And yes, 16.04 would be good.

[dannon]

@carlfeberhard This looks like a good fix to me, I like it. Do you want this applied to 16.04 as well? If so, would you rather retarget this PR or cherrypick the change back?

[carlfeberhard]

@nsoranzo @dannon No, it's meant for 16.07. This isn't a fix for a bug, it's a workaround/enhancement for the corner case where a client is misconfigured. 16.04 is frozen and things that are central like this need more time to shake out before they're in production IMO.

@mvdbeek Glad it's working.

[martenson]

thanks @carlfeberhard

[dannon]

Cool, just wanted to check what your intent was.