Correct MIN_FINAL_CLTV_EXPIRY to match our enforced requirements

Our enforced requirements for HTLC acceptance is that we have at
least HTLC_FAIL_BACK_BUFFER blocks before the HTLC expires. When we
receive an HTLC, the HTLC would be "already expired" if its
`cltv_expiry` is current-block + 1 (ie the next block could
broadcast the commitment transaction and time out the HTLC). From
there, we want an extra HTLC_FAIL_BACK_BUFFER in blocks, plus an
extra block or two to account for any differences in the view of
the current height before send or while the HTLC is transiting the
network.

lightning/src/ln/channelmanager.rs

@@ -578,7 +578,10 @@ pub(super) const CLTV_FAR_FAR_AWAY: u32 = 6 * 24 * 7; //TODO?
 /// Minimum CLTV difference between the current block height and received inbound payments.
 /// Invoices generated for payment to us must set their `min_final_cltv_expiry` field to at least
 /// this value.
-pub const MIN_FINAL_CLTV_EXPIRY: u32 = HTLC_FAIL_BACK_BUFFER;
+// Note that we fail if exactly HTLC_FAIL_BACK_BUFFER + 1 was used, so we need to add one for
+// any payments to succeed. Further, we don't want payments to fail if a block was found while
+// a payment was being routed, so we add an extra block to be safe.
+pub const MIN_FINAL_CLTV_EXPIRY: u32 = HTLC_FAIL_BACK_BUFFER + 3;
 
 // Check that our CLTV_EXPIRY is at least CLTV_CLAIM_BUFFER + ANTI_REORG_DELAY + LATENCY_GRACE_PERIOD_BLOCKS,
 // ie that if the next-hop peer fails the HTLC within