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/wtmock/client_db.go

@@ -231,7 +231,6 @@ func (m *ClientDB) listClientSessions(tower *wtdb.TowerID,
 		if tower != nil && *tower != session.TowerID {
 			continue
 		}
-		session.Tower = m.towers[session.TowerID]
 		sessions[session.ID] = &session
 
 		if cfg.PerAckedUpdate != nil {