This solves the following issues:

* unlike lex->m_sql_cmd and lex->sql_command, thd->query_plan_flags
  is not reset in Prepared_statement::execute, it survives
  till the log_slow_statement(), so slow logging behaves correctly in --ps

* using thd->query_plan_flags for both slow_log_filter and
  log_slow_admin_statements means the definition of "admin" statements
  for the slow log is the same no matter how it is filtered out.