Payment channel API, client suggests channel time window

Lets the client request the duration of a payment channel. Servers can accept or deny the request.
The client's version message now takes an optional time window parameter. It defaults to 24h - 1 minute.
The server will by default allow channels to have 4 h to 1 week time window. There is a hard limit of 2 h on the server minimum time window, causing a IllegalArgumentsException, since it takes about 2 h to close the channel.
If you put in some work to avoid changing these numbers and make clients handle old servers, you can probably avoid the protocol version bump, though, to my knowledge essentially no one is using this stuff in prod yet, so I dont mind a protocol version bump personally.