@param http the http sever on which to set the callback
@param path the path for which to invoke the callback
@param cb the callback function that gets invoked on requesting path
@param cb_arg an additional context argument for the callback
@return 0 on success, -1 if the callback existed already, -2 on failure
Set a callback for a specified URI
@param http the http sever on which to set the callback @param path the path for which to invoke the callback @param cb the callback function that gets invoked on requesting path @param cb_arg an additional context argument for the callback @return 0 on success, -1 if the callback existed already, -2 on failure