Improve logging of autovacuum I/O activity

This adds some I/O stats to the logging of autovacuum (when the
operation takes long enough that log_autovacuum_min_duration causes it
to be logged), so that it is easier to tune.  Notably, it adds buffer
I/O counts (hits, misses, dirtied) and read and write rate.

Authors: Greg Smith and Noah Misch