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/errors.go

@@ -20,10 +20,6 @@ var (
 	// down.
 	ErrNegotiatorExiting = errors.New("negotiator exiting")
 
-	// ErrNoTowerAddrs signals that the client could not be created because
-	// we have no addresses with which we can reach a tower.
-	ErrNoTowerAddrs = errors.New("no tower addresses")
-
 	// ErrFailedNegotiation signals that the session negotiator could not
 	// acquire a new session as requested.
 	ErrFailedNegotiation = errors.New("session negotiation unsuccessful")