lnd: introduce separate key-value payment db

We also addd this new db on the server level to use it in the
following commit to do all the payment related queries of the
rpcserver.
We add a new payment db instance on the server level. Which we
will you for the payment related queries in a following commit.

server.go

@@ -335,6 +335,12 @@ type server struct {
 
 	invoicesDB invoices.InvoiceDB
 
+	// kvPaymentsDB is the DB that contains all functions for managing
+	// payments.
+	//
+	// TODO(ziggie): Replace with interface.
+	kvPaymentsDB *channeldb.KVPaymentsDB
+
 	aliasMgr *aliasmgr.Manager
 
 	htlcSwitch *htlcswitch.Switch
@@ -678,6 +684,7 @@ func newServer(ctx context.Context, cfg *Config, listenAddrs []net.Addr,
 		addrSource:     addrSource,
 		miscDB:         dbs.ChanStateDB,
 		invoicesDB:     dbs.InvoiceDB,
+		kvPaymentsDB:   dbs.KVPaymentsDB,
 		cc:             cc,
 		sigPool:        lnwallet.NewSigPool(cfg.Workers.Sig, cc.Signer),
 		writePool:      writePool,
@@ -1127,9 +1134,7 @@ func newServer(ctx context.Context, cfg *Config, listenAddrs []net.Addr,
 		PathFindingConfig:   pathFindingConfig,
 	}
 
-	paymentControl := channeldb.NewKVPaymentsDB(dbs.ChanStateDB)
-
-	s.controlTower = routing.NewControlTower(paymentControl)
+	s.controlTower = routing.NewControlTower(dbs.KVPaymentsDB)
 
 	strictPruning := cfg.Bitcoin.Node == "neutrino" ||
 		cfg.Routing.StrictZombiePruning