Compatibility test for PostgreSQL 13.0, documentation improvements
- Performed a compatibility test for the new PostgreSQL v13.0 - could be used with the postgreSQL plugin
- Added a better error handling for misconfigured PostgreSQL configurations - if the processing unit could not connect to the external database server
- Added a section "error handling" to the postgreSQL plugin documentation: How to detect the problem if the system could not come up as there is no database connection
- Added a HA overview using the postgreSQL plugin and the mendelson AS2 to the documentation