Make PaymentChannel.ServerConnection.paymentIncrease asynchronous.

When I implemented a payment channel server with akka, I would have good use of having the server side paymentIncrease method to be asynchronous. This a proposal.
I'm note sure if the addCallback should use an explicit exciter coming from the PaymentChannelServerConstructor, defaulting to SameThreadExecutor. For now I left it.
This is the same thing as Futures.immediateFuture