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 has a new structure to support
two semantic concepts being a LTTng event and a JUL logger that have a
relation of N:1. This addition does not impact the current interfaces of
the agent.

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