internal/router: bulk metadata API implemention

We also fix all paths so that they start with / after splitting. This makes them compatible with the usual way that paths are specified in URLs.
Although writer is never used in "serveMeta", it does break the common "w, r" api with all other "serve*" functions. Not a problem if we are sure, that the code will never use writers.
Also the comment is not fully understood - why not just change the code if it is required?