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.
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