[#169] pool: Close inner pools during close routine

Some apps do not reuse pool instance and expect that
`pool.Close()` free resources. But it didn't actually
close inner SDK clients, so it leads to goroutine leak
in storage.

Signed-off-by: Alex Vanin <a.vanin@yadro.com>

pool/mock_test.go

@@ -189,3 +189,7 @@ func (m *mockClient) restartIfUnhealthy(ctx context.Context) (healthy bool, chan
 	}
 	return
 }
+
+func (m *mockClient) close() error {
+	return nil
+}

pool/pool.go

@@ -78,6 +78,8 @@ type client interface {
 	dial(ctx context.Context) error
 	// see clientWrapper.restartIfUnhealthy.
 	restartIfUnhealthy(ctx context.Context) (bool, bool)
+	// see clientWrapper.close.
+	close() error
 }
 
 // clientStatus provide access to some metrics for connection.
@@ -1080,6 +1082,13 @@ func (c *clientWrapper) incRequests(elapsed time.Duration, method MethodIndex) {
 	}
 }
 
+func (c *clientWrapper) close() error {
+	if c.client != nil {
+		return c.client.Close()
+	}
+	return nil
+}
+
 func (c *clientStatusMonitor) handleError(ctx context.Context, st apistatus.Status, err error) error {
 	if err != nil {
 		if needCountError(ctx, err) {
@@ -2766,6 +2775,13 @@ func (p *Pool) NetworkInfo(ctx context.Context) (netmap.NetworkInfo, error) {
 func (p *Pool) Close() {
 	p.cancel()
 	<-p.closedCh
+
+	// close all clients
+	for _, pools := range p.innerPools {
+		for _, cli := range pools.clients {
+			_ = cli.close()
+		}
+	}
 }
 
 // SyncContainerWithNetwork applies network configuration received via