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This reverts commit 26897301
and adds some changes.

Only a table on which the 10.3 ADD COLUMN...LAST was performed
can "lose its instantness" when it becomes empty. After
instant DROP COLUMN or reordering columns, we cannot simply
convert the table to the canonical format, because the data
dictionary cache and all possibly existing references to it
from other client connection threads would have to be adjusted.

rec_init_offsets_comp_ordinary(): Remove a FIXME comment.

row_log_allocate(): Replace dict_index_t::is_instant()
with a more appropriate condition that ignores dict_table_t::instant.