updated protocol to match new lightning server

[amaiberg]

The old server was using ints for the session ids whereas the new one uses string hashes. This updates the client to adhere to the new server protocol.

[mathisonian]

Thanks for all the updates @amaiberg! I have just a few updates and will merge these

[mathisonian]

https://github.com/amaiberg/lightning-scala/blob/newserver/src/main/scala/org/viz/lightning/Lightning.scala#L66

comparison to -1 should be updated to check for empty string now

[mathisonian]

https://github.com/amaiberg/lightning-scala/blob/newserver/src/test/scala/org/viz/lightning/LightningSuite.scala#L10

assert should be updated to ensure string is not empty