zpay32: Change min_final_cltv_expiry_delta.

[ziggie1984]

We already do not allow a min_final_cltv_expiry_delta for invoices lower than 18 and default to 80 when not specified:

https://github.com/lightningnetwork/lnd/blob/master/lnrpc/invoicesrpc/addinvoice.go#L364-L380.

Lately during a chat with CDecker we found out that CLN will not always provide a min_cltv_expiry_delta in their invoice. In that case we would default to 9 when paying the invoice which hower CLN would reject because the delta is too low for them.

So this PR is mostly to smoothen out some interoperability issues between CLN<=>LND

For more infos see: https://github.com/lightning/bolts/blob/master/02-peer-protocol.md#cltv_expiry_delta-selection

Hmm looking at https://github.com/lightningnetwork/lnd/blob/master/lncfg/config.go we might analyse all our constants because I think we are still at an old spec level.

For example:

// DefaultIncomingBroadcastDelta defines the number of blocks before the
// expiry of an incoming htlc at which we force close the channel. We
// only go to chain if we also have the preimage to actually pull in the
// htlc. BOLT #2 suggests 7 blocks.

But I think now the value is also at 18:

the deadline for received HTLCs this node has fulfilled: the deadline after which the channel has to be failed and the HTLC fulfilled on-chain before its cltv_expiry. See steps 4-7 above, which imply a deadline of 2R+G+S blocks before cltv_expiry: 18 blocks is reasonable.

what do you think ?

[Roasbeef]

what do you think ?

Most implementations should accept a higher value, so I don't think the change should cause interop issues.

Looks like this change (from 9 to 18) was made 3 years ago: lightning/bolts#785

[Roasbeef]

I think we should also make sure to note this in the release notes as well, just to potentially find some edge case during the rc1 process.

[Roasbeef]

LGTM 🎒

[yyforyongyu]

Pending a release note, otherwise LGTM🙏

[ziggie1984]

Connor kept the value at 9 because of backwards compatibility (lightning/bolts#785 (comment)). But since we have not allowed invoice creation with lower values than 18 blocks since 0.11 I think its a good time to change the values.

EDIT: Also changed the DefaultIncomingBroadcastDelta from 10 to 18 according to the spec. I don't think this causes compatibility issues but let's first see whether all the test pass.

[yyforyongyu]

LGTM👍

[murchandamus]

Hey, just noticed a typo while writing the Optech mention for this PR:

Suggested change

	This only effects external invoices which do not supply the
	This only affects external invoices which do not supply the