Fix: remove JUL loglevel filtering from the agent

The loglevel filtering is now done on the UST side setup by
liblttng-ctl. To achieve that, the agent had to be refactored to support
two semantic concept being a lttng event and a JUL logger that have a
relation of N:1.

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