This is to support enabling all events with different loglevels in two
different sessions.
For this, if any loglevel have been defined, the 'int_loglevel' filter
is added to the UST event. The liblttng-ust-jul library has been
modified to stop filtering loglevel in the agent.
This commit adds two tests, one for a back to back session that are
destroyed and a second one for multi loglevel session.
Signed-off-by: David Goulet <dgoulet@efficios.com>