Add basic postgres collectd metrics

- query_plans collects the numbers of various table scans and returned tuples of the user tables
- queries collects the numbers of various table modifications (i. e. insertions, updates, deletions) of the user tables
- table_states collects the numbers of live and dead rows in the user tables
- disk_io collects disk block access counts for user tables
Merge pull request #557 from alphagov/feature/postgres-replication-monitoring

Postgres monitoring