Clarify roles in failing HTLCs, implement most of it#13

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TheBlueMatt merged 2 commits intolightningdevkit:masterfrom

TheBlueMatt:2018-03-htlc-failures

Mar 22, 2018

Commits on Mar 20, 2018

Fix length requirement in peer_handler msg decode
TheBlueMatt
committed

Commits on Mar 22, 2018

Clarify roles in failing HTLCs, impl it, support rebalances
TheBlueMatt
committed