_withValueIn() results eventually show QL.timeout error

Summary: The problem is pretty clear: we are caching an InvocationValue, which makes sense in principle, but in practice that InvocationValue includes its start time, which eventually causes a timeout.

Specifically, the problem is in Invocation.flatMap(), which does this bit:

      val timeProvider = ecology.api[querki.time.TimeProvider]
      if (timeProvider.now.getMillis > inv.context.endTime.getMillis) {
        Future.failed(PublicException("QL.timeout"))
      } else ...

Sadly, the implication is that _withValueIn() needs improvement. The heart of the problem is that caching an InvocationValue like this is corrupt -- we can't do that. We're going to have to come up with a cleverer way to do this, such that we cache something more appropriate. (Probably a Future, I suspect.)