expose sql.DB to let orm operate on it (#1015)

* expose sql.DB to let orm operate on it

* add missing RawDB methods

* add NewSqlConnFromDB for cooperate with dtm

core/stores/sqlx/tx.go

@@ -71,7 +71,7 @@ func begin(db *sql.DB) (trans, error) {
 }
 
 func transact(db *commonSqlConn, b beginnable, fn func(Session) error) (err error) {
-	conn, err := getSqlConn(db.driverName, db.datasource)
+	conn, err := db.connProv()
 	if err != nil {
 		logInstanceError(db.datasource, err)
 		return err