wireproto: make wirepeer look-before-you-leap on batching This means that users of request batching don't need to worry themselves with capability checking. Instead, they can just use batching, and if the remote server doesn't support batching for some reason the wirepeer code will transparently un-batch the requests. This will allow for some slight simplification in a handful of places. Prior to this change, largefiles would have been silently broken against a server which did not support batching.