lnd: plug in graph SQL migration into dev build

This commit plugs in the graph kvdb-to-sql migration for builds
containing the `test_native_sql` tag. This will allow us to perform
local tests and write itests for the migration without exposing it to
the production release build.

config_prod.go

@@ -3,9 +3,13 @@
 package lnd
 
 import (
+	"context"
+
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/sqldb"
+	"github.com/lightningnetwork/lnd/sqldb/sqlc"
 )
 
 // getGraphStore returns a graphdb.V1Store backed by a graphdb.KVStore
@@ -16,3 +20,15 @@ func (d *DefaultDatabaseBuilder) getGraphStore(_ *sqldb.BaseDB,
 
 	return graphdb.NewKVStore(kvBackend, opts...)
 }
+
+// getSQLMigration returns a migration function for the given version.
+//
+// NOTE: this is a no-op for the production build since all migrations that are
+// in production will also be in development builds, and so they are not
+// defined behind a build tag.
+func getSQLMigration(ctx context.Context, version int,
+	kvBackend kvdb.Backend,
+	chain chainhash.Hash) (func(tx *sqlc.Queries) error, bool) {
+
+	return nil, false
+}