compressed secondary index pages after attempting the insert.

btr_cur_update_alloc_zip(): Reset the insert buffer free bits if there is
insufficient space available on secondary index pages.  Do not update
the bits after successful operation.

btr_cur_update_in_place(): Update the free bits in the index buffer.

btr_cur_optimistic_update(): First check rec_offs_any_extern(), because
checking it is cheaper than checking the "external storage" flags of the
update vector.  Update the insert buffer free bits in any case after
successfully calling btr_cur_update_alloc_zip().