Integration with Nimbus Go wrapper

[pedropombeiro]

This PR provides support for starting a Nimbus node and leveraging it to send/receive messages. It uses Nix to build the Nimbus wrapper API.

It also allows disabling geth by not including it in the build process.

Closes #123

Next steps would be to integrate Nimbus in status-react (without envelope monitor or mailserver functionality):

• generalize StatusNode to use facades over the actual node implementation (tricky since there are many supporting types from geth):
  • plug in call to nimbus_start in StatusNode.StartWithOptions (in this case, we'd not register mailserversService)
  • add missing configuration options to nimbus_start (e.g. bootstrap and static nodes, etc.)
  • implement RPC functionality between Nimbus and status-go and abstract it behind a facade
  • implement facade over service management API (gethNode.Service(serviceInstance))
    • implement support for service management in Nimbus bridge
    • bridge with respective services in Nimbus (wallet, ShhExt, accounts manager)
• Add an argument to params.NodeConfig to determine the type of node to use (this would be passed from the UI in status-react).
  • Add UI in status-react to determine the ETH node type.

cc @adambabik for next steps validation. Will probably set up a meeting to discuss it.

[pedropombeiro]

Moved the geth node code to a separate file so we can compile the console client without the geth node.

[pedropombeiro]

Ping @adambabik and @cammellos for review, so we can get this merged.