New reference for PSR/Cache discussion

[FGM]

Just to gain insights on other cache API designs, Dropbox just open-sourced its internal libraries, and they include a caching package, described here:

https://godoc.org/github.com/dropbox/godropbox/caching

(yes it's Go, not PHP, that's part of looking elsewhere)

[Larry Garfield]

Thanks!

The key difference I see is that their flush() is "Delete all", whereas
we're calling it "clear", and we have "call set multiple times" rather
than "call setMultiple". I think I'm happy with our model at this point.

I will be doing final polish of the Cache PSR and handing it over to
Paddy for the final Review round as soon as I have a few ours blocked
out to do so. :-)

--Larry Garfield