page_zip_max_ins_size(): New function.

btr_cur_optimistic_insert(), btr_cur_optimistic_delete(),
btr_page_split_and_insert(), btr_compress(): Do not update the
ibuf free bits for non-leaf pages or pages belonging to a clustered index.
The insert buffer only covers operations on leaf pages of secondary indexes.
For pages covered by the insert buffer, limit the max_ins_size to
page_zip_max_ins_size().

buf_page_get_gen(): Merge the insert buffer after decompressing the page.

buf_page_io_complete(): Relax the assertion about ibuf_count.  For
compressed-only pages, the insert buffer merge takes place
in buf_page_get_gen().

ibuf_index_page_calc_free_bits(), ibuf_index_page_calc_free_from_bits(),
ibuf_index_page_calc_free(), ibuf_update_free_bits_if_full(),
ibuf_update_free_bits_low(), ibuf_update_free_bits_for_two_pages_low(),
ibuf_set_free_bits_low(): Add the parameter zip_size.  Limit the maximum
insert size to page_zip_max_ins_size().