When an online ALTER TABLE rebuilds the table, we would get false
duplicate key errors on row_log_table_apply() if the initial table
scan performed READ UNCOMMITTED. If we perform REPEATABLE READ, the
log will never be older than what we copied from the table, and the
false duplicates will be gone.

prepare_inplace_alter_table_dict(): Assign a consistent read view for
online table-copying ALTER TABLE.

BTR_CREATE_SAME_PK_FLAG: Remove.

row_ins_duplicate_online_is_newer(): Remove. It is simpler to
construct old record versions in the table scan phase than in the log
application phase.

row_ins_duplicate_online(): Simplify.

row_ins_duplicate_error_in_clust_online(): Wrapper for
row_ins_duplicate_online(). Check also cursor->up_match for a
duplicate.

row_merge_read_clustered_index(): When rebuilding the table online,
perform a REPEATABLE READ instead of READ UNCOMMITTED.

row_purge_upd_exist_or_extern_func(): Refuse the operation if an
off-page column would be freed.

row_purge_record_func(): Only refuse the operation if an off-page
column would be freed. Do process other undo log records for tables
that are being altered online.

row_vers_build_for_consistent_read(): Correct comments and remove
redundant code.

trx_undo_get_undo_rec_low(): Correct a comment.

rb:1438 approved by Jimmy Yang