rpc: refactor WSClient initialisation

Fix the following linter warning:
```
pkg/rpc/client/wsclient.go:99:18  govet  copylocks: literal copies lock value from *cl: github.com/nspcc-dev/neo-go/pkg/rpc/client.Client contains sync.RWMutex
```

pkg/rpc/client/client.go

@@ -83,10 +83,19 @@ type calculateValidUntilBlockCache struct {
 // New returns a new Client ready to use. You should call Init method to
 // initialize network magic the client is operating on.
 func New(ctx context.Context, endpoint string, opts Options) (*Client, error) {
-	url, err := url.Parse(endpoint)
+	cl := new(Client)
+	err := initClient(ctx, cl, endpoint, opts)
 	if err != nil {
 		return nil, err
 	}
+	return cl, nil
+}
+
+func initClient(ctx context.Context, cl *Client, endpoint string, opts Options) error {
+	url, err := url.Parse(endpoint)
+	if err != nil {
+		return err
+	}
 
 	if opts.DialTimeout <= 0 {
 		opts.DialTimeout = defaultDialTimeout
@@ -110,19 +119,17 @@ func New(ctx context.Context, endpoint string, opts Options) (*Client, error) {
 	//	if opts.Cert != "" && opts.Key != "" {
 	//	}
 
-	cl := &Client{
+	cl.ctx = ctx
-		ctx:      ctx,
+	cl.cli = httpClient
-		cli:      httpClient,
+	cl.endpoint = url
-		endpoint: url,
+	cl.cache = cache{
-		cache: cache{
 		nativeHashes: make(map[string]util.Uint160),
-		},
-		latestReqID: atomic.NewUint64(0),
 	}
+	cl.latestReqID = atomic.NewUint64(0)
 	cl.getNextRequestID = (cl).getRequestID
 	cl.opts = opts
 	cl.requestF = cl.makeHTTPRequest
-	return cl, nil
+	return nil
 }
 
 func (c *Client) getRequestID() uint64 {

pkg/rpc/client/wsclient.go

@@ -79,20 +79,13 @@ const (
 // You should call Init method to initialize network magic the client is
 // operating on.
 func NewWS(ctx context.Context, endpoint string, opts Options) (*WSClient, error) {
-	cl, err := New(ctx, endpoint, opts)
-	if err != nil {
-		return nil, err
-	}
-
-	cl.cli = nil
-
 	dialer := websocket.Dialer{HandshakeTimeout: opts.DialTimeout}
 	ws, _, err := dialer.Dial(endpoint, nil)
 	if err != nil {
 		return nil, err
 	}
 	wsc := &WSClient{
-		Client:        *cl,
+		Client:        Client{},
 		Notifications: make(chan Notification),
 
 		ws:            ws,
@@ -102,6 +95,13 @@ func NewWS(ctx context.Context, endpoint string, opts Options) (*WSClient, error
 		requests:      make(chan *request.Raw),
 		subscriptions: make(map[string]bool),
 	}
+
+	err = initClient(ctx, &wsc.Client, endpoint, opts)
+	if err != nil {
+		return nil, err
+	}
+	wsc.Client.cli = nil
+
 	go wsc.wsReader()
 	go wsc.wsWriter()
 	wsc.requestF = wsc.makeWsRequest