Allow Accepting HTLCs with less value than the onion claims to pay

[TheBlueMatt]

If so configured, an LSP may skim some funds off the top of an HTLC, but we really dont want them to mess with the onion. Thus, we need a way to override the amount the onion claims to pay so that we can accept an HTLC that has an onion that says it pays X, but only pays X-$(configured fee). I'm not exactly sure how best to let the user configure $(configured fee), but ignoring it entirely isn't super practical as we need some way to figure out if an MPP payment is complete or not.

TODOs:

• Allow forwarding less than the amount in the onion v1 (Allow forwarding less than the amount in the onion  #2319)
• Include the counterparty skimmed fee in PaymentClaimed
• Link to LSPS2: JIT Channels. BitcoinAndLightningLayerSpecs/lsp#22 when it's merged

[TheBlueMatt]

Slipping to 116 :(

[valentinewallace]

cc lightning/blips#25

[TheBlueMatt]

The main parts of this are done, we really just need to include the skimmed fee in PaymentClaimed now, which can wait till 117.