eth: Added GPO to suggest default gas prices

This PR fixes a regression of the RPC where the default gas price that
was being used for transaction wasn't properly using the GPO. This PR
adds the GPO back to suggest gas prices rather than the hardcoded
default of 10000000000000.

Closes #2194

eth/backend.go

@@ -269,7 +269,7 @@ func (s *Ethereum) APIs() []rpc.API {
 		}, {
 			Namespace: "eth",
 			Version:   "1.0",
-			Service:   NewPublicTransactionPoolAPI(s.TxPool(), s.Miner(), s.ChainDb(), s.EventMux(), s.BlockChain(), s.AccountManager()),
+			Service:   NewPublicTransactionPoolAPI(s),
 			Public:    true,
 		}, {
 			Namespace: "eth",