page_zip_set_alloc(): New function, to configure zlib to use a memory heap.

btr_store_big_rec_extern_fields(): Reduce memLevel, so that a heap of
256 kilobytes will suffice.  Use page_zip_set_alloc().

btr_copy_externally_stored_field_prefix_low(): Use page_zip_set_alloc().

page_zip_compress(), page_zip_decompress(): Use page_zip_set_alloc().