Instrument freezing in autovacuum log reports.

Instrument freezing in autovacuum log reports.

Add a new line to log reports from autovacuum (as well as VACUUM VERBOSE
output) that shows information about freezing.  Emphasis is placed on
the total number of heap pages that had one or more tuples frozen by
VACUUM.  The total number of tuples frozen is also shown.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Jeff Janes <jeff.janes@gmail.com>
Discussion: https://postgr.es/m/CAH2-WznTY6D0zyE8VLrC6Gd4kh_HGAXxnTPtcOQOOsxzLx9zog@mail.gmail.com