innobase_drop_foreign_try(): Don't evict and reload the dict_foreign_t
during instant ALTER TABLE if the FOREIGN KEY constraint is being
dropped.
The MDEV-19630 fix (commit 07b1a26c33)
was incomplete, because it did not cover a case where the
FOREIGN KEY constraint is being dropped.
which are pointed to the table being altered
Problem:
========
InnoDB failed to change the column name present in foreign key cache
for instant add column. So it leads to column mismatch for the consecutive
rename of column.
Solution:
=========
Evict the foreign key information from cache and load the foreign
key information again for instant operation.
MDEV-11369 (instant ADD COLUMN) introduced a regression in the case the
leftmost leaf page of the clustered index is empty except for the
hidden metadata record.
btr_pcur_store_position(): If the only record in the leftmost leaf page
is the metadata record, store the position before the first user record
in the tree.
