new syntax for RequiredValidUntil filter

[Rod Widdowson]

> I guess one question: what are we gaining here? The issue isn't really that the current setting doesn't take durations, but that
it's an outlier
> in taking a numeric value as seconds?

Two answers
 - The default value of nothing meaning do nothing sucks.  This doesn't need a change, it just sucks
 - It is an edge case but it has bitten us more than once (we misremember in a support call or even in a bug fix that "this is the
funky outlier").  We need to move to a position in which durations much be durations or milliseconds and the only way to make that
happen and not break people is to migrate through a changed setting.

I'll happily entertain either ways of fixing this (like "just change it to a duration in V4 and break people who didn't migrate when
they got the warning to move away from seconds").   I didn't get any feedback in the case so I just went ahead with my proposal,
doesn't mean it cannot easily be backed out.

That latter suggestion of mine has the advantage of not forking the federation's documentation which does feel like a big win.
Maybe that's the thing to do?

R