Make `Striped.custom` public about guava HOT 3 OPEN

I feel bad about this (#2514). It's one of those changes that's significant enough to require our relatively heavyweight review process but not widely used enough to clearly justify the effort :( Hopefully someday, but it's probably going to have to wait until we make a proper push on long-standing feature requests.

from guava.

It seems to me - perhaps naively - that making a package private, static method public would be change small enough to fly under the radar pretty smoothly. Then again I'm the first to admit that I lack knowledge about your review process so I'm not going to make an argument :).

It is a shame however: this is a very useful class and making it open for extension would make it even more useful (I've encountered the need for this in two completely separate real-world projects).

from guava.