watchtower: load persisted towers upon TowerClient creation

We do this as a convenience for WatchtowerClient users so that they do
not need to re-add towers upon restarts. We ensure not to re-add towers
that have been previously removed by determining whether it has any
lingering active sessions.

watchtower/wtdb/tower.go

@@ -1,6 +1,8 @@
 package wtdb
 
 import (
+	"encoding/hex"
+	"fmt"
 	"io"
 	"net"
 
@@ -92,6 +94,15 @@ func (t *Tower) LNAddrs() []*lnwire.NetAddress {
 	return addrs
 }
 
+// String returns a user-friendly identifier of the tower.
+func (t *Tower) String() string {
+	pubKey := hex.EncodeToString(t.IdentityKey.SerializeCompressed())
+	if len(t.Addresses) == 0 {
+		return pubKey
+	}
+	return fmt.Sprintf("%v@%v", pubKey, t.Addresses[0])
+}
+
 // Encode writes the Tower to the passed io.Writer. The TowerID is not
 // serialized, since it acts as the key.
 func (t *Tower) Encode(w io.Writer) error {