Explicit name of round tripper field

After more thinking I agree with the comment in commit https://github.com/couchbaselabs/cbfs/commit/0debf6483d6204c246839d5e043c8a624bcc799a . It makes more sense to explicitly name roundtripper field to keep the semantics of "next" rountripper clear. This seems to be more about behavior, not about structure. Sorry to mess with this lately.