pages becoming empty.  Remove work-arounds for empty pages.  This fixes
Issue #82.

ibuf_get_volume_buffered(): Add the output parameter n_recs, for returning
the minimum number of records on the page.

ibuf_insert_low(): Refuse to buffer IBUF_OP_DELETE if the page could
become empty.

btr_page_get_father_node_ptr(): Remove the work-around for the page
being empty.

page_zip_dir_delete(): Revert to the version from branches/zip.
Always invoke page_zip_clear_rec().