REBUILD PARTITION under LOCK TABLE

Collapsed patch including updates from the reviews.

In case of failure in ALTER ... PARTITION under LOCK TABLE
the server could crash, due to it had modified the locked
table object, which was not reverted in case of failure,
resulting in a bad table definition used after the failed
command.

Solved by instead of altering the locked table object and
its partition_info struct, creating an internal temporary
intermediate table object used for altering,
just like the non partitioned mysql_alter_table.
So if an error occur before the alter operation is complete,
the original table is not modified at all.
But if the alter operation have succeeded so far that it
must be completed as whole,
the table is properly closed and reopened.
(The completion on failure is done by the ddl_log.)