[#2048] network/cache: Optimize ClientCache

1. Remove a layer of indirection for mutex, `ClientCache` is already
   used by pointer.
2. Fix duplication of a `AllowExternal` field.

Signed-off-by: Evgenii Stratonikov <evgeniy@morphbits.ru>

pkg/network/cache/client.go

@@ -13,10 +13,9 @@ type (
 	// ClientCache is a structure around neofs-sdk-go/client to reuse
 	// already created clients.
 	ClientCache struct {
-		mu            *sync.RWMutex
+		mu      sync.RWMutex
 		clients map[string]*multiClient
 		opts    ClientCacheOpts
-		allowExternal bool
 	}
 
 	ClientCacheOpts struct {
@@ -32,17 +31,15 @@ type (
 // `opts` are used for new client creation.
 func NewSDKClientCache(opts ClientCacheOpts) *ClientCache {
 	return &ClientCache{
-		mu:            new(sync.RWMutex),
 		clients: make(map[string]*multiClient),
 		opts:    opts,
-		allowExternal: opts.AllowExternal,
 	}
 }
 
 // Get function returns existing client or creates a new one.
 func (c *ClientCache) Get(info clientcore.NodeInfo) (clientcore.Client, error) {
 	netAddr := info.AddressGroup()
-	if c.allowExternal {
+	if c.opts.AllowExternal {
 		netAddr = append(netAddr, info.ExternalAddressGroup()...)
 	}
 	cacheKey := string(info.PublicKey())