The ALTER related code cannot do at the same time both:
- modify partitions
- change column data types

Explicit changing of a column data type together with a partition change is
prohibited by the parter, so this is not allowed and returns a syntax error:

  ALTER TABLE t MODIFY ts BIGINT, DROP PARTITION p1;

This fix additionally disables implicit data type upgrade
(e.g. from "MariaDB 5.3 TIME" to "MySQL 5.6 TIME", or the other way
around according to the current mysql56_temporal_format) in case of
an ALTER modifying partitions, e.g.:

  ALTER TABLE t DROP PARTITION p1;

In such commands now only the partition change happens, while
the data types stay unchanged.

One can additionally run:

  ALTER TABLE t FORCE;

either before or after the ALTER modifying partitions to
upgrade data types according to mysql56_temporal_format.