When nodes get invalidated in the cache, they have to be recomputed. Let the IDE know which of the nodes are pending by sending `Api.ExpressionUpdate.Payload.Pending` message.

# Important Notes
This PR introduces new `Api.ExpressionUpdate.Payload.Pending` message. This message is delivered before re-computation of nodes. Later `Api.ExpressionUpdate.Payload.Value` or other is sent to notify the IDE that a value for given node is available.

Trivial implementation of of the `Api.ExpressionUpdate.Payload.Pending` message in the IDE is provided by this PR to (improperly) visualize pending node status - further improvements needed in follow up PRs.