JUL: use root logger to capture events

The JUL agent now uses the root logger ("") to capture all events. This
allows us to remove the Timer thread and cleanup a huge portion of the
code base. It simplifies a great deal the internal structure of the
agent since we don't have to monitor the Logger object anymore.

Since tracepoint filtering is done in UST, we just the LTTng handler to
the root logger and send everything to UST.

Signed-off-by: David Goulet <dgoulet@efficios.com>