* When no I/O activity has been reported in pg_statio_all_tables for a
database, the queries resulted in a division by zero. Cange this to a
NULL instead. Still a problem for Zabbix, but maybe a little cleaner.
* Add ability to specify the sslmode parameter when connecting to
PostgreSQL
* Fix min versions for replication queries
* Add query-tests target to main Makefile
* Add a mode to test all metric queries
* Add a script to run query tests against different versions of
PostgeSQL
* Add Docker elements for query testing
* Switch to using a --config flag when specifying the config file
* Fix some metric queries
* Allow the agent address to be configured
* Allow the sslmode connection parameter to be configured
