Allow visibility into the holding cell via PendingHTLCsForwardable

[tnull]

Depending on the source/destination of HTLCs it might not make sense to delay forwarding at all (as there wouldn't be any privacy benefits under certain circumstances in practice). We should allow users to decide whether or not to apply the forwarding delay depending on the HTLCs in the holding cell.

[MaxFangX]

👍 This would allow our LSP to use a shorter forwarding delay (or none at all) if the HTLC originates from a Lexe user or is forwarded to a Lexe user, as there is no privacy benefit in these cases which means we can optimize for payment latency. We'd just add the delay when our LSP forwards payments between non-Lexe nodes.

[TheBlueMatt]

What circumstances are you thinking about here? We currently delay forwarding and receiving each by one event cycle. If an LSP wants to not delay payments I'm not sure them delaying forwards but not payments is really worth it.