Refactor nbtinsert.c so that the final itemsz of each new non-pivot
tuple (the MAXALIGN()'d size) is determined once.  Most of the functions
used by leaf page inserts used the insertstate.itemsz value already.
This commit makes everything use insertstate.itemsz as standard
practice.  The goal is to decouple tuple size from "effective" tuple
size.  Making this distinction isn't truly necessary right now, but that
might change in the future.

Also explain why we consistently apply MAXALIGN() to get an effective
index tuple size.  This was rather unclear, in part because it isn't
actually strictly necessary right now.