eth, xeth: fix GasPriceOracle goroutine leak

XEth.gpo was being initialized as needed. WithState copies the XEth
struct including the gpo field. If gpo was nil at the time of the copy
and Call or Transact were invoked on it, an additional GPO listenLoop
would be spawned.

Move the lazy initialization to GasPriceOracle instead so the same GPO
instance is shared among all created XEths.

Fixes #1317
Might help with #1930

xeth/xeth.go

@@ -59,24 +59,8 @@ const (
 	LogFilterTy
 )
 
-func DefaultGas() *big.Int { return new(big.Int).Set(defaultGas) }
-
-func (self *XEth) DefaultGasPrice() *big.Int {
-	if self.gpo == nil {
-		self.gpo = eth.NewGasPriceOracle(self.backend)
-	}
-	return self.gpo.SuggestPrice()
-}
-
 type XEth struct {
-	backend  *eth.Ethereum
-	frontend Frontend
-
-	state   *State
-	whisper *Whisper
-
-	quit          chan struct{}
-	filterManager *filters.FilterSystem
+	quit chan struct{}
 
 	logMu    sync.RWMutex
 	logQueue map[int]*logQueue
@@ -92,16 +76,18 @@ type XEth struct {
 
 	transactMu sync.Mutex
 
-	agent *miner.RemoteAgent
-
-	gpo *eth.GasPriceOracle
+	// read-only fields
+	backend       *eth.Ethereum
+	frontend      Frontend
+	agent         *miner.RemoteAgent
+	gpo           *eth.GasPriceOracle
+	state         *State
+	whisper       *Whisper
+	filterManager *filters.FilterSystem
 }
 
 func NewTest(eth *eth.Ethereum, frontend Frontend) *XEth {
-	return &XEth{
-		backend:  eth,
-		frontend: frontend,
-	}
+	return &XEth{backend: eth, frontend: frontend}
 }
 
 // New creates an XEth that uses the given frontend.
@@ -118,6 +104,7 @@ func New(ethereum *eth.Ethereum, frontend Frontend) *XEth {
 		transactionQueue: make(map[int]*hashQueue),
 		messages:         make(map[int]*whisperFilter),
 		agent:            miner.NewRemoteAgent(),
+		gpo:              eth.NewGasPriceOracle(ethereum),
 	}
 	if ethereum.Whisper() != nil {
 		xeth.whisper = NewWhisper(ethereum.Whisper())
@@ -207,6 +194,12 @@ func cTopics(t [][]string) [][]common.Hash {
 	return topics
 }
 
+func DefaultGas() *big.Int { return new(big.Int).Set(defaultGas) }
+
+func (self *XEth) DefaultGasPrice() *big.Int {
+	return self.gpo.SuggestPrice()
+}
+
 func (self *XEth) RemoteMining() *miner.RemoteAgent { return self.agent }
 
 func (self *XEth) AtStateNum(num int64) *XEth {