tapfreighter: bump feerate to min relay fee

If the fee estimation returns a fee rate lower than the min relay fee,
we should use the min relay fee instead. This commit implements this
behavior for the tapfreighter.

tapfreighter/chain_porter.go

@@ -1128,6 +1128,17 @@ func (p *ChainPorter) stateStep(currentPkg sendPackage) (*sendPackage, error) {
 				return nil, fmt.Errorf("unable to estimate "+
 					"fee: %w", err)
 			}
+			minRelayFee, err := p.cfg.Wallet.MinRelayFee(ctx)
+			if err != nil {
+				return nil, fmt.Errorf("unable to obtain "+
+					"minimum relay fee: %w", err)
+			}
+
+			// If the fee rate is below the minimum relay fee, we'll
+			// bump it up.
+			if feeRate < minRelayFee {
+				feeRate = minRelayFee
+			}
 		}
 
 		readableFeeRate := feeRate.FeePerKVByte().String()