Avoid capturing onValue closure in useSubscription forever

[fsoikin]

An issue similar to #purescript-elmish/83

The useSubscription hook was directly subscribing the onValue callback to the subscription in init, thus capturing that particular instance of onValue forever, ignoring the fact that onValue may change from call to call.

To fix, made it work similar to useEffect - sending an actual message from the subscription and calling onValue from update.

Added a test to check that it happens correctly. While I was at it, added a similar test for useEffect.

[mcordova47]

Nice, thanks for adding the tests!