Hyperledger Besu

**Discussed in**  Node meeting 23.4.2020.

**Description** 
Try out `Hyperledger Besu` as Ethereum Client for `go-perun`.

**Context** 
`go-perun` already works with `gnache-cli`, `geth`, `parity` and `Infura`.
`Besu` will hopefully work out-of-the-Box too.

**Acceptance criteria** 
A complete walkthrough of the `perun` 2-party-payment protocol is possible.
All on-chain balances are updated accordingly.

**Hints** 
- Setup `Besu` either using a premade [build](https://besu.hyperledger.org/en/stable/Tutorials/Examples/Private-Network-Example/) or compile from [source](https://wiki.hyperledger.org/display/BESU/Building+from+source#running-developer-builds).
- Fund two addresses that will be used by `Alice` and `Bob`.
- Check out `297-payment-demo`, open `alice.yaml`&`bob.yaml` and set:
- chain url to the address of the `Besu` client.
- secret keys to the funded accounts from above.
- Set `Bob` and `Alice` addresses in `network.yaml`
- Start up `Bob` and let him deploy the contracts.
- Set the addresses of the deployed contracts in `alice.yaml` and start `Alice`.
- Play throught the protocol of connecting/opening/sending/closing a channel.
- Verify on-chain balances.

[Demo readme](https://git.perun.network/perun/go-perun/-/blob/297-payment-demo/perun/README.md).