Simplify state managed by VACUUM.

Simplify state managed by VACUUM.

Reorganize the state struct used by VACUUM -- group related items
together to make it easier to understand.  Also stop relying on stack
variables inside lazy_scan_heap() -- move those into the state struct
instead.  Doing things this way simplifies large groups of related
functions whose function signatures had a lot of unnecessary redundancy.

Switch over to using int64 for the struct fields used to count things
that are reported to the user via log_autovacuum and VACUUM VERBOSE
output.  We were using double, but that doesn't seem to have any
advantages.  Using int64 makes it possible to add assertions that verify
that the first pass over the heap (pruning) encounters precisely the
same number of LP_DEAD items that get deleted from indexes later on, in
the second pass over the heap.  These assertions will be added in later
commits.

Finally, adjust the signatures of functions with IndexBulkDeleteResult
pointer arguments in cases where there was ambiguity about whether or
not the argument relates to a single index or all indexes.  Functions
now use the idiom that both ambulkdelete() and amvacuumcleanup() have
always used (where appropriate): accept a mutable IndexBulkDeleteResult
pointer argument, and return a result IndexBulkDeleteResult pointer to
caller.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-By: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CAH2-WzkeOSYwC6KNckbhk2b1aNnWum6Yyn0NKP9D-Hq1LGTDPw@mail.gmail.com