postgresql_default.conf, postgresql plugin: Split default queries by category.

postgresql_default.conf, postgresql plugin: Split default queries by category.

The default queries have been split as follows:

 | old            | new                                |
 +----------------+------------------------------------+
 | database       | backends, transactions             |
 | user_tables    | queries, query_plans, table_states |
 | io_user_tables | disk_io                            |

This allows for a more fine-grained control over which statistics are to be
collected.

The documentation and default queries definition have been updated to reflect
this change.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>