Terminate method

Michael, I figured you'd want a diff that's easy to look at, so here's the pull request.

- I didn't add a WT_EXTRACTOR.terminate method, did you want one?

- I'm no longer using ```wiredtiger_extension_terminate()``` for any purpose, and, I don't see much call for it in most applications because the handle.terminate method will be sufficient.   That said, I figured it couldn't hurt to leave it in?