sim-node: validate channel policy and enforce unique scid

[carlaKC]

Add some validation to make sure that we get sane channels in, specifically:

• Policies that have valid max in flight / htlc size (this would be caught by our sanity checks, but this catches it early)
• No repeated channel ids, this will break other things (and can't be true irl)

[bjohnson5]

LGTM! I tested a few invalid channel policies and scids and the correct errors were thrown. Left one totally optional suggestion.