Use PageIndexTupleOverwrite() within nbtsort.c

Attached patch slightly simplifies nbtsort.c by making it use
PageIndexTupleOverwrite() to overwrite the last right non-pivot tuple
with the new high key (pivot tuple). PageIndexTupleOverwrite() is
designed so that code like this doesn't need to delete and re-insert
to replace an existing tuple.

This slightly simplifies the code, and also makes it marginally
faster. I'll add this to the 2019-09 CF.

-- 
Peter Geoghegan