Don't assume that Log4j and JUL share the same log level mappings

Don't assume that Log4j and JUL share the same log level mappings

We explicitly set the log level of Log4j events to
LTTNG_LOGLEVEL_LOG4J_ALL instead of relying on the fact that
LTTNG_LOGLEVEL_LOG4J_ALL and LTTNG_LOGLEVEL_JUL_ALL are mapped to the
same value.

The resulting additional branch does not seem to incur a significant
performance penalty and, as such, is deemed acceptable.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>