Do not assume anything about the contents of the bitmap.

ibuf_update_free_bits_low(): Use this function only for uncompressed pages.
Remove the parameter zip_size.  This function avoids latching the bitmap page
and updating the bitmap when the bits do not change.

ibuf_update_free_bits_zip(): New function based on ibuf_update_free_bits_low(),
for use on compressed pages.  Remove the parameter max_insert_size that
was used for computing the before image of the free bits.  Always update the
bitmap.

ibuf_index_page_calc_free_zip(): New function, factored out from
ibuf_index_page_calc_free().

ibuf_update_free_bits_if_full(): Document that this function must only be
invoked on uncompressed pages.  Add a debug assertion about this.