txpool: set default GasCeil from to 0

This PR is some improvments based on: PR#2680
If the node did not set `Eth.Miner` in config.toml, the default GasCeil
will be 30000000, it is not an accurate number for both BSC mainnet and testnet.
Set it to 0 means disable the transaction GasLimit check, i.e. the TxPool will
accept transactions with any GasLimit size.

miner/miner.go

@@ -62,7 +62,7 @@ type Config struct {
 
 // DefaultConfig contains default settings for miner.
 var DefaultConfig = Config{
-	GasCeil:  30000000,
+	GasCeil:  0,
 	GasPrice: big.NewInt(params.GWei),
 
 	// The default recommit time is chosen as two seconds since