Count and log pages set all-frozen by vacuum

Vacuum currently counts and logs the number of pages of a relation
with newly frozen tuples. It does not count the number of pages newly
set all-frozen in the visibility map.

The number of pages set all-frozen in the visibility map by a given
vacuum can be useful when analyzing which normal vacuums reduce the
number of pages future aggressive vacuum need to scan.

Also, empty pages that are set all-frozen in the VM don't show up in
the count of pages with newly frozen tuples. When making sense of the
result of visibilitymap_summary() for a relation, it's helpful to know
how many pages were set all-frozen in the VM by each vacuum.

The attached patch set makes visibilitymap_set() return the old value
of the block's VM bits and then uses that to increment a new counter
in the LVRelState.

Here is example logging output with the patch:

create table foo (a int, b int) with (autovacuum_enabled = false);
insert into foo select generate_series(1,1000), 1;
delete from foo where a > 50;
vacuum (freeze, verbose) foo;

finished vacuuming "melanieplageman.public.foo": index scans: 0
pages: 4 removed, 1 remain, 5 scanned (100.00% of total)
tuples: 950 removed, 50 remain, 0 are dead but not yet removable
frozen: 1 pages from table (20.00% of total) had 50 tuples frozen. 5
pages set all-frozen in the VM

- Melanie