Light node - investigation

Investigate the solution space for implementing a light node and define a feature proposal.

We investigated light clients a while ago. Conceptually light nodes are similar to light clients. The light node is interested in the most recent state (while only keeping limited history), while a light client is interested in the most recent state of an account or smart contract.

An overview on light nodes is found in [https://docs.google.com/document/d/1sACTjSbOOjrmCVOI-qrYek5-Ui1u7jEu936zIGQNYrk/edit#heading=h.4t87c52nxqdn](https://docs.google.com/document/d/1sACTjSbOOjrmCVOI-qrYek5-Ui1u7jEu936zIGQNYrk/edit#heading=h.4t87c52nxqdn)

In addition we have:

* [Simple Light Client](https://docs.google.com/document/d/1ZNDi2wIiGu6xd0J1r2rNXqXijzyJKz6Mpe1AL-vQQKU/edit?usp=drive_link) concept
* [Simple Light Client](https://docs.google.com/presentation/d/1SImqPEUbhz7jWX05kMXxpsVtDaP5gckysLOdzNT8RBI/edit?usp=sharing) presentation

Start by reading the linked light-node overview and the Simple Light Client concept document and presentation. Compare the light-node and light-client goals described in the issue, then produce a feature proposal that defines the solution space and the intended light-node behavior.

Written by the indexing model from the issue text.