1. _bt_compare fixed to work properly with new code in _bt_insertonpg

   (old _bt_compare always returned >= 0 while comparing with P_HIKEY
   on root page - it breaks root page when _bt_insertonpg tries insert
   new minimal key into root page).
2. Fixed bug concerns "empty" pages: non-rightmost pages with only P_HIKEY
   present on it. Such pages appear after vacuum.