tapgarden: 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 minting transaction.

tapgarden/planter.go

@@ -566,6 +566,17 @@ func (c *ChainPlanter) fundGenesisPsbt(ctx context.Context,
 			return nil, fmt.Errorf("unable to estimate fee: %w",
 				err)
 		}
+		minRelayFee, err := c.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
+		}
 
 		log.Infof("estimated fee rate for batch: %x, %s",
 			batchKey[:], feeRate.FeePerKVByte().String())