There are still some bugs in the code.

btr_store_big_rec_extern_fields(): Remove assertion on dict_table_is_zip()
to ease testing.

btr_free_externally_stored_field(): Test page_zip instead of
dict_table_is_zip().

page_zip_write_rec(): Add parameter "create".  Try to handle externally
stored columns.

rec_offs_any_extern(): Correct the function comment.

Add rec_offs_n_extern() and page_zip_get_n_prev_extern().

page_zip_dir_decode(): Replace assertion with if (...) return(FALSE).

page_zip_decompress(): Do not clear page_zip->n_blobs after counting the
BLOBs.

page_zip_write_blob_ptr(): Use page_zip_get_n_prev_extern().
Correct an off-by-one error in memcpy().