Invalidate txs with below base-fee-floor gas-price in txpool

Fixes #291

Before, the transaction pool accepted transactions that have a
gas-price below the configured base-fee-floor, which would never be
executed.

Now, the transaction pool rejects those transactions immediately -
giving the submitting user an immediate response and better UX.

core/txpool/celo_validation.go

@@ -1,6 +1,7 @@
 package txpool
 
 import (
+	"errors"
 	"math/big"
 
 	"github.com/ethereum/go-ethereum/common"
@@ -9,6 +10,10 @@ import (
 	"github.com/ethereum/go-ethereum/params"
 )
 
+// ErrGasPriceDoesNotExceedBaseFeeFloor is returned if the gas price specified is
+// lower than the configured base-fee-floor
+var ErrGasPriceDoesNotExceedBaseFeeFloor = errors.New("gas-price is less than the base-fee-floor")
+
 // AcceptSet is a set of accepted transaction types for a transaction subpool.
 type AcceptSet = map[uint8]struct{}
 
@@ -56,5 +61,16 @@ func CeloValidateTransaction(tx *types.Transaction, head *types.Header,
 		return exchange.ErrUnregisteredFeeCurrency
 	}
 
+	celoGasPrice, err := exchange.ConvertCurrencyToCelo(
+		currencyCtx.ExchangeRates,
+		tx.FeeCurrency(),
+		tx.GasFeeCap(),
+	)
+	if err != nil {
+		return err
+	}
+	if new(big.Int).SetUint64(opts.Config.Celo.EIP1559BaseFeeFloor).Cmp(celoGasPrice) == 1 {
+		return ErrGasPriceDoesNotExceedBaseFeeFloor
+	}
 	return nil
 }