Implement off-chain downstream to on-chain upstream incoming HTLC claiming

[ariard]

See TODO in ChannelManager

[TheBlueMatt]

Specifically, this TODO:

						// TODO: There is probably a channel monitor somewhere that needs to
						// learn the preimage as the channel already hit the chain and that's
						// why it's missing.

[ariard]

@valentinewallace closed by #611 ?

[valentinewallace]

Yup, I think that's right @ariard