Refactor JUL to agent namespace

The specific JUL concept has been removed and it is now "agent" objects.
This is the first step to support multiple agents within the same
subsystem.

Signed-off-by: David Goulet <dgoulet@efficios.com>
Add the support for multiple agent

For this, a new field has been added to the register message, the domain
type. Agent object and agent application object have now a domain type
so we can enable/disable the right event for the right agent for a
client command or during an application registration.

The "jevent" name has been renamed to "aevent" so further remove the
concept of Java event.

Signed-off-by: David Goulet <dgoulet@efficios.com>
Change --jul-port-tcp and jul.port to use agent namespace

They are both renamed to --agent-port-tcp and "agent.port" file.

Signed-off-by: David Goulet <dgoulet@efficios.com>
Adding log4j agent support

This adds the log4j domain to the public ABI and "-l, --log4j" option in
the lttng command line tool to be able to use it.

Note that at this commit, the JUL default channel and event name are
used for the log4j domain. That might become specific to the domain in
the long run.

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