[#567] network/clients: Implement method to close the cached clients

Update API Go library with introduce `Client.Conn` method. Implement
`ClientCache.CloseAll` method which reads and closes connections of all
cached clients.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/network/cache/client.go

@@ -75,3 +75,21 @@ func (c *ClientCache) Get(netAddr *network.Address) (client.Client, error) {
 
 	return cli, nil
 }
+
+// CloseAll closes underlying connections of all cached clients.
+//
+// Ignores closing errors.
+func (c *ClientCache) CloseAll() {
+	c.mu.RLock()
+
+	{
+		for _, cl := range c.clients {
+			con := cl.Conn()
+			if con != nil {
+				_ = con.Close()
+			}
+		}
+	}
+
+	c.mu.RUnlock()
+}