externally stored columns.  Make some use of the fact in compression and
decompression.

rec_init_offsets(): Add ut_ad(dict_index_is_clust(index)) before flagging
external columns.

page_zip_get_trailer_len(): Assert ut_ad(!page_zip->n_blobs) unless
dict_index_is_clust(index).

page_zip_get_n_prev_extern(): Add ut_ad(dict_index_is_clust(index)).

page_zip_compress(), page_zip_decompress_low(): Add dict_index_is_clust()
assertions.  Allow completely external storage of columns.  Simplify
the computation of "externs".

page_zip_write_rec(): Store leaf node records of non-clustered indexes
with one memcpy(), avoiding rec_offs_n_extern() and the for loop.

page_zip_write_blob_ptr(): Add ut_ad(dict_index_is_clust(index)).
Simplify the computation of "externs".

page_zip_clear_rec(): Only attempt to zero out BLOB pointers when
the record belongs to a leaf page of a clustered index.

page_zip_dir_delete(): Take a fast path when the record is
not on a leaf page of a clustered index.
Simplify the computation of "externs".

page_zip_copy(): Assert that n_blobs is zero on anything else than
leaf pages of clustered indexes.