Initialize block.page.zip only once.

PageConverter::update(): Initialize m_page_zip_ptr
as late as possible.
(We should really remove it at some point.)

PageConverter::operator(): Refer to block->page.zip instead of
m_page_zip_ptr.

AbstractCallback::get_frame(): Define static. Refer
to block->page.zip.data directly.

fil_iterate(): Refer to block->page.zip.data directly.

fil_tablespace_iterate(): Initialize block.page.zip.data as soon
as possible.