Assert that _bt_binsrch() binary searches with scantid set in insertion
scankey cannot be performed on leaf pages.  Leaf-level binary searches
where scantid is set must use _bt_binsrch_insert() instead.

_bt_binsrch_insert() is likely to have additional responsibilities in
the future, such as searching within GIN-style posting lists using
scantid.  It seems like a good idea to tighten things up now.