Also: extend failover logging configuration with file-based handler to make
sure that the user gets _some_ logs even if they do not configure Lacre at
all.
- Add a new test input message for a new test identity, test scenario
configuration and a test key.
- While retrieving message payload, determine charset based on the
Content-Type header. When missing, default to UTF-8.
- Use more comprehensible variables names.
- Adjust logging levels.