TC3: Balance logic in plugins

[dino-rodriguez]

Ethereum

• Balance logic in ilp-plugin-ethereum
• Balance logic for Ethereum peering (might be covered in the above case, but no Ethereum links are being used yet, so this is untested)

Lightning/Bitcoin

• Balance logic in plugin ilp-plugin-lightning (WIP)
• Balance logic for Lightning peering (same as Ethereum)

XRP

• Balance logic ilp-plugin-xrp-asym-server
• Balance logic in ilp-plugin-xrp-asym-client
• Balance logic in ilp-plugin-xrp-paychan

A possible solution to explore is to move the balance logic down to BTP. This would make BTP stateful though, if we want to persist balances.

This should be done before TC2.