[#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>

go.mod

@@ -13,7 +13,7 @@ require (
 	github.com/multiformats/go-multiaddr v0.3.1
 	github.com/nspcc-dev/hrw v1.0.9
 	github.com/nspcc-dev/neo-go v0.95.0
-	github.com/nspcc-dev/neofs-api-go v1.26.2-0.20210528133202-6cd349738889
+	github.com/nspcc-dev/neofs-api-go v1.26.2-0.20210531071300-89be8d3f5ada
 	github.com/nspcc-dev/neofs-crypto v0.3.0
 	github.com/nspcc-dev/neofs-sdk-go v0.0.0-20210520210714-9dee13f0d556
 	github.com/nspcc-dev/tzhash v1.4.0

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()
+}