Add support for Fedora's state-tokens for optimistic locking.

[tpendragon]

We'll probably have to keep around the old implementation for backwards compatibility, but if an X-State-Token exists we should use it for locking to avoid the issue with If-Unmodified-Since only being resolvable to 1 second.

See specification here: https://fedora.info/2018/11/22/spec/#state-tokens

Should only work in FCRepo 5: https://jira.lyrasis.org/browse/FCREPO-2976?jql=text%20~%20%22X-State-Token%22