core: txpool listen for ChainHeadEvent instead of ChainEvent

Changed the transaction pool to listen for ChainHeadEvent when resetting
the state instead of ChainEvent. It makes very little sense to burst
through transactions while we are catching up (e.g., have more than one
block to process)

core/transaction_pool.go

@@ -65,7 +65,7 @@ func NewTxPool(eventMux *event.TypeMux, currentStateFn stateFn, gasLimitFn func(
 		gasLimit:     gasLimitFn,
 		minGasPrice:  new(big.Int),
 		pendingState: state.ManageState(currentStateFn()),
-		events:       eventMux.Subscribe(ChainEvent{}, GasPriceChanged{}),
+		events:       eventMux.Subscribe(ChainHeadEvent{}, GasPriceChanged{}),
 	}
 	go pool.eventLoop()
 
@@ -80,7 +80,7 @@ func (pool *TxPool) eventLoop() {
 		pool.mu.Lock()
 
 		switch ev := ev.(type) {
-		case ChainEvent:
+		case ChainHeadEvent:
 			pool.resetState()
 		case GasPriceChanged:
 			pool.minGasPrice = ev.Price