Add Duration and Supplier<Duration> overloads for Supplier#memoizeWithExpiration

[not-solis]

Directly addresses #3688

I don't want to step on any toes since I saw that long, TimeUnit overloads are in the cards for several classes, but I also see the use in a Duration supplier, so I went and wrote this up real quick so we can at least have it in the works.

One thing I noticed while writing this is that com.google.common.util.concurrent.Internal#toNanosSaturated is package-scoped internal, but the only method in there is applicable to all legacy API's that require a long, TimeUnit, and there are several scattered in base, cache, and collect under com.google.common. It might be worth considering gauging the need for it to public-scoped, if at some point the human race figures out how to prolong life indefinitely.

Also first contribution here, hope all looks good :)

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[not-solis]

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[jbduncan]

Hi @mfboulos, welcome to Guava. :)

I think it's worth mentioning that it's unlikely this PR will be merged, because the Guava team prefer to do API reviews internally (which is apparently the hard part) and then write the code themselves to match whatever API they've agreed on.

See the contribution guidelines for more information.

[jbduncan]

That being said, I'm sure that this code would be a great start for @chriskilding's needs, who opened #3688, if you're happy for them to take the code as-is. :)

[kluever]

It might be worth considering gauging the need for it to public-scoped

FWIW, we have a copy of this in our (still internal) com.google.common.time package, which we hope to get around to releasing at some point.

[not-solis]

Understandable! My intuition was to put this PR in there just to have the code handy for you guys, but I understand there's more discussion behind it since it's essentially an extension to the existing API.

[cpovirk]

We added the Duration overload recently, including under Android. I don't know that we'll end up adding a Supplier<Duration> overload at all, but we can come back to this PR if we someday resume the discussion on #3688 and decide to go for it.