[#1157] network/cache: Optimize client fetch from multiClient

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

pkg/network/cache/multi.go

@@ -235,16 +235,21 @@ func (x *multiClient) RawForAddress(addr network.Address, f func(client *rawclie
 }
 
 func (x *multiClient) client(addr network.Address) clientcore.Client {
-	x.mtx.Lock()
-
 	strAddr := addr.String()
 
+	x.mtx.RLock()
 	c, cached := x.clients[strAddr]
+	x.mtx.RUnlock()
+
+	if !cached {
+		x.mtx.Lock()
+
+		c, cached = x.clients[strAddr]
 		if !cached {
 			c = x.createForAddress(addr)
 		}
-
 		x.mtx.Unlock()
+	}
 
 	return c
 }