The logging backends supported by the default `LogFactoryImpl` have all
reached their end-of-life (except JUL and `SimpleLog`). Third-generation
logging backends, such as Log4j Core, support multiple logger contexts
per application and therefore can not be handled by the simplified
caching mechanism in `LogFactoryImpl`.
This PR introduces two new `LogFactory` implementations,
`Log4j2LogFactory` and `Slf4jLogFactory`, that forward messages to the
Log4j API and SLF4J.
During initialization the three existing factories are checked in the
following order:
1. the `Log4j2LogFactory` has highest priority, since the Log4j API can
faithfully transmit messages of type `Object`,
2. the `Slf4jLogFactory` is the next choice. However, if the
Log4j-to-SLF4J bridge is present we log directly through SLF4J
instead of the Log4j API.
3. the legacy `LogFactoryImpl` has lowest priority.
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