miner: Remove need for dcrd in benchmark mode.

This makes it possible to run gominer in benchmark mode (-B) without
having setup a dcrd instance.  This is particularly useful when
comissioning new machines or testing the software, so that users do not
have to setup an entire simnet environment or wait for a full mainnet
dcrd and dcrwallet sync.

miner.go

@@ -128,6 +128,12 @@ func newSoloMiner(ctx context.Context, devices []*Device) (*Miner, error) {
 	return m, nil
 }
 
+func newBenchmarkMiner(devices []*Device) (*Miner, error) {
+	return &Miner{
+		devices: devices,
+	}, nil
+}
+
 func NewMiner(ctx context.Context) (*Miner, error) {
 	workDone := make(chan []byte, 10)
 
@@ -139,6 +145,10 @@ func NewMiner(ctx context.Context) (*Miner, error) {
 		return nil, fmt.Errorf("no devices started")
 	}
 
+	if cfg.Benchmark {
+		return newBenchmarkMiner(devices)
+	}
+
 	var m *Miner
 	if cfg.Pool == "" {
 		m, err = newSoloMiner(ctx, devices)