vacuumlazy.c: Rename dead_tuples to dead_items.

Commit 8523492 simplified what it meant for an item to be considered
"dead" to VACUUM: TIDs collected in memory (in preparation for index
vacuuming) must always come from LP_DEAD stub line pointers in heap
pages, found following pruning.  This formalized the idea that index
vacuuming (and heap vacuuming) are optional processes.  Unlike pruning,
they can be delayed indefinitely, without any risk of that violating
fundamental invariants.  For example, leaving LP_DEAD items behind
clearly won't add to the risk of transaction ID wraparound.  You can't
have transaction ID wraparound without transaction IDs.  Renaming
anything that references DEAD tuples (tuples with storage) reinforces
all this.

Code outside vacuumlazy.c continues to fudge the distinction between
dead/deleted tuples, and LP_DEAD items.  This is necessary because
autovacuum scheduling is still mostly driven by "dead items/tuples"
statistics.  In the future we may find it useful to replace this model
with something more sophisticated, as a step towards teaching autovacuum
to perform more frequent vacuuming that targeting individual indexes
that happen to be more prone to becoming bloated through version churn.

In passing, simplify some function signatures that deal with VACUUM's
dead_items array.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/CAH2-WzktGBg4si6DEdmq3q6SoXSDqNi6MtmB8CmmTmvhsxDTLA@mail.gmail.com

Author: petergeoghegan