Several race conditions between MDEV-15562 instant ALTER TABLE and purge
were observed.

The most obvious race condition resulted in a reported assertion failure in
dict_index_t::instant_add_field(): instant.n_core_fields == n_core_fields
would not hold if the table was emptied by purge after the time
dict_table_t::prepare_instant() was called.

During purge, it can turn out that the table is logically empty, only
containing a metadata record. If the metadata record is of the type
created by MDEV-11369 instant ADD COLUMN, it can be removed and
dict_index_t::clear_instant_add() can be called. This will convert the
table to the canonical non-instant format. (If the metadata record is
of the MDEV-15562 type, then it can only be deleted if the table becomes
empty as the result of rollback of an instant ALTER TABLE operation.)

row_purge_remove_clust_if_poss_low(): Add a debug check that ensures
that purge can never remove a MDEV-15562 metadata record.

ha_innobase::open(): Add a comment about the necessity of rolling
back any recovered instant ALTER TABLE transaction on the table.

instant_metadata_lock(): An auxiliary function to acquire a page latch
on the metadata record, to prevent race conditions.

dict_table_t::prepare_instant(), dict_index_t::instant_add_field(),
dict_table_t::rollback_instant(), innobase_instant_try():
Invoke instant_metadata_lock() in order to prevent race conditions.

dict_index_t::instant_add_field(): Correct debug assertions.
The == was guaranteed by code in dict_table_t::prepare_instant()
that was introduced in MDEV-15562. Due to the race condition,
we could occasionally have <=, but never >= like the code was
after MDEV-11369.

ha_innobase_inplace_ctx::instant_column(): Wrapper for
dict_table_t::instant_column(). Add debug assertions.