add support for bulk imports with COPY FROM STDIN

- db.Prepare("COPY foo (a, b) FROM STDIN") returns a COPY IN statement
- stmt.Exec(a, b) inserts new items
- stmt.Exec() syncs the input stream, since sql.Stmt.Close() does not
  return errors from stmt.Close()
optimize escapeText for strings without \r, \n, \t or \