Problem:
=======
 - There is a race condition between purge and rollback of alter
operation. Alter rollback marks the index as corrupted.
At the same time, purge is working on the same index and leads
to assert failure. This is caused by
commit 7c0b9c60 (MDEV-15250).

Solution:
=======
 - After MDEV-15250, InnoDB logs the operation only at the
end of transaction commit and applies the log in
ha_innobase::commit_inplace_alter_table() and
also via dml thread. So there is no need for purge to work
on uncommitted index.

The assertion would fail in the test innodb.innodb-index-online
when the following call is added to the start of the function
row_purge_remove_sec_if_poss_leaf():
if (!index->is_committed()) sleep(5);