Wip xattr spillout

This branch enables the previously-merged xattr spillout functionality, which had been (1) erroneously disabled due to never setting the flag on file creates and (2) otherwise buggy due to not copying the spillout xattr on clones.
It also includes removal of the user_only param from the FileStore interfaces.

It passed a teuthology suite run and looks as good as anything else on the gitbuilders.