of clustered indexes.  Previously, parts of the code assumed that these
columns would exist on all leaf pages.  Simplify the update-in-place of
these columns.

Add inline function dict_index_is_clust() to replace all tests
index->type & DICT_CLUSTERED.

Remove the redo log entry types MLOG_ZIP_WRITE_TRX_ID and
MLOG_ZIP_WRITE_ROLL_PTR, because the modifications to these columns
are covered by logical logging.

Fuse page_zip_write_trx_id() and page_zip_write_roll_ptr() into
page_zip_write_trx_id_and_roll_ptr().

page_zip_dir_add_slot(), page_zip_available(): Add flag "is_clustered",
so that no space will be reserved for TRX_ID and ROLL_PTR on leaf pages
of secondary indexes.

page_zip_apply_log(): Flag an error when val==0 is encoded with two bytes.

page_zip_write_rec(): Add debug assertions that there is enough space
available for the entry before copying the data bytes of the record.