watchtower: make use of the new AddressIterator

This commit upgrades the wtclient package to make use of the new
`AddressIterator`. It does so by first creating new `Tower` and
`ClientSession` types. The new `Tower` type has an `AddressIterator`
instead of a list of addresses. The `ClientSession` type contains a
`Tower`.

watchtower/wtclient/interface.go

@@ -118,3 +118,50 @@ type ECDHKeyRing interface {
 	// key.
 	DeriveKey(keyLoc keychain.KeyLocator) (keychain.KeyDescriptor, error)
 }
+
+// Tower represents the info about a watchtower server that a watchtower client
+// needs in order to connect to it.
+type Tower struct {
+	// ID is the unique, db-assigned, identifier for this tower.
+	ID wtdb.TowerID
+
+	// IdentityKey is the public key of the remote node, used to
+	// authenticate the brontide transport.
+	IdentityKey *btcec.PublicKey
+
+	// Addresses is an AddressIterator that can be used to manage the
+	// addresses for this tower.
+	Addresses AddressIterator
+}
+
+// NewTowerFromDBTower converts a wtdb.Tower, which uses a static address list,
+// into a Tower which uses an address iterator.
+func NewTowerFromDBTower(t *wtdb.Tower) (*Tower, error) {
+	addrs, err := newAddressIterator(t.Addresses...)
+	if err != nil {
+		return nil, err
+	}
+
+	return &Tower{
+		ID:          t.ID,
+		IdentityKey: t.IdentityKey,
+		Addresses:   addrs,
+	}, nil
+}
+
+// ClientSession represents the session that a tower client has with a server.
+type ClientSession struct {
+	// ID is the client's public key used when authenticating with the
+	// tower.
+	ID wtdb.SessionID
+
+	wtdb.ClientSessionBody
+
+	// Tower represents the tower that the client session has been made
+	// with.
+	Tower *Tower
+
+	// SessionKeyECDH is the ECDH capable wrapper of the ephemeral secret
+	// key used to connect to the watchtower.
+	SessionKeyECDH keychain.SingleKeyECDH
+}