Sync using full node's web socket

[tdroxler]

Once the explorer-backend is up-to-date with the sync, we could maybe use the websocket to sync new blocks to avoid polling.

Very low priority as it can bring more complexity and anyway our polling of 5 seconds almost got a new block every time. Bu it might be good to explore this one day.

[tdroxler]

#616 PR is there, but needs some heavy testing