Support configuring relation summary stats off

For databases with very large numbers of relations, these queries can be
prohibitively expensive to run every minute, so offer a way to turn them
off by setting `relation_stats: False`. (Still on by default, as very
large relation counts are not the typical case.)
Document relation_stats config param on PostgreSQL

... and rephrase the `superuser` config param, and illustrate them with
a single example.
Merge pull request #148 from gthb/patch-2

Support configuring relation summary stats off