RouterClient.call should just return function return

Currently, `RouterClient.call` returns `tuple[float, Any]`, with a server-provided timestamp along with the object returned by the remote function. While it's useful to have the timestamp to know if the received value is old or something, maybe the client should just deal with it and not provide it to the user.

There could be a normal `call -> Any` method, and a `call_with_timestamp: tuple[float, Any]` to provide it upon request. Maybe also the client could have a configurable recency threshold, and compare the age of the received response.