Create a function to generate a unique SCID

[JssDWt]

In the proposed JIT channel spec a client gets assigned a unique SCID that is used to identify a payment as a JIT channel request.

There's a few options:

1. Keep track of all SCIDs of open and pending channels and of pending JIT channel requests and make sure the new SCID does not yet exist.
2. Reserve a range of numbers for JIT channel requests, so only JIT channel requests can have a SCID in that range. That way there's only JIT channel SCIDs to track.

Either way, the SCIDs have to be globally unique throughout the node. They have to be persisted on creation. So upon generating one, it has to be reserved until the expiration time.

[JssDWt]

Closing: SCIDs are considered unique when generating, because the number space is so large.