page_zip_dir_delete() will need to handle BLOBs.

rec_set_field_extern_bits(), rec_set_field_extern_bits_new():
Add parameter offsets.

rec_offs_set_nth_extern(): New function to set an extern bit in offsets.
This will be called when an extern bit is set in a record.

page_cur_rec_insert(), page_cur_insert_rec_low(): Document that the
parameter "offsets" is in/out.

page_zip_dir_delete(): Note that the array of BLOB pointers will need
to be shifted.

page0zip.ic: Document the entry type for clearing a record.

page_zip_available(): Add parameter "index".  Remove parameters
"is_leaf" and "is_clustered".

page_zip_get_trailer_len(): New function for computing the trailer length
of the compressed page.

page_zip_apply_log(): Implement the modification log entry type for
clearing the data bytes of a record.

page_zip_decompress(): Initialize n_blobs when actually copying the
BLOB pointers to place.

page_zip_validate(): Add diagnostic messages for failures.  Check
also m_start, m_end, and n_blobs.

page_zip_write_blob_ptr(): Add page_zip_validate() assertion.