- maybe_fulfill_htlc returns a forwarding callback that
covers both cases.
- previously, the callback of hold invoices was called as a
side-effect of lnworker.check_mpp_status.
- the same data structures (lnworker.trampoline_forwardings,
lnworker.trampoline_forwarding_errors) are used for both
trampoline forwardings and hold invoices.
- maybe_fulfill_htlc still recursively calls itself to perform
checks on trampoline onion. This is ugly, but ugliness is now
contained to that method.
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