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>