- Wait until HTLCs are irrevocably removed before cleaning up their
data structures (MPP and forwarding)
- keep methods maybe_cleanup_mpp and maybe_cleanup_forwarding separate
- perform cleanup in htlc_switch, so that process_unfulfilled_htlc
has less side effects
- In htlc_switch, we blank the onion_packet_hex field to signal that
an HTLC has been processed. An item of chan.unfulfilled_htlcs may
go through 4 stages:
- 1. not forwarded yet: (None, onion_packet_hex)
- 2. forwarded: (forwarding_key, onion_packet_hex)
- 3. processed: (forwarding_key, None), not irrevocably removed yet
- 4. done: (forwarding_key, None), irrevocably removed
- in test_lnpeer, an extra iteration of htlc_switch has been added to
trampoline forwarding tests
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