Make _bt_keep_natts_fast() use datum_image_eq().

Make _bt_keep_natts_fast() use datum_image_eq().

An upcoming patch that adds deduplication to the nbtree AM will rely on
_bt_keep_natts_fast() understanding that differences in TOAST input
state can never affect its answer.  In particular, two opclass-equal
datums (with opclasses deemed safe for deduplication) should never be
treated as unequal by _bt_keep_natts_fast() due to TOAST input
differences.

This also seems like a good idea on general principle.  nbtsplitloc.c
will now occasionally make better decisions about where to split a leaf
page.  The behavior of _bt_keep_natts_fast() is now somewhat closer to
the behavior of _bt_keep_natts().

Discussion: https://postgr.es/m/CAH2-Wzn3Ee49Gmxb7V1VJ3-AC8fWn-Fr8pfWQebHe8rYRxt5OQ@mail.gmail.com