Fix issue #10 (super read only does not prevent DROP TRIGGER)

- add super read only handling to mysql_create_and_drop_trigger;
- tweak error message in trans_begin to note whether regular or super
  read only is effective;
- add MTR tests for the above changes and for multi-table update which
  was implemented but not covered in MTR.

Tested by running super_read_only_basic on a local build. The same patch has undergone Percona Server Jenkins CI testing.