multi: move payment related code into own package

This commit moves most of the code into its own package. It is
the smallest code move possible without moving import cycles and
keeping the changes to the code base as small as possible during
refactor.

payments/db/setup_test.go

@@ -0,0 +1,11 @@
+package paymentsdb
+
+import (
+	"testing"
+
+	"github.com/lightningnetwork/lnd/kvdb"
+)
+
+func TestMain(m *testing.M) {
+	kvdb.RunTests(m)
+}