Add LOG4J2 domain to the Log4j 2.x agent

This commit adds a new LOG4J2 domain with native Log4j 2.x loglevels in
addition to the existing LOG4J domain. Both domains can be used
individually or at the same time by instantiating one or more appenders.

For example, a single appender in Log4j 2.x native mode:

    <?xml version="1.0" encoding="UTF-8"?>
    <Configuration status="WARN">
      <Appenders>
        <Lttng name="LTTNG" domain="LOG4J2"/>
      </Appenders>
      <Loggers>
        <Root level="all">
          <AppenderRef ref="LTTNG"/>
        </Root>
      </Loggers>
    </Configuration>

Two appenders to enable both domains:

    <?xml version="1.0" encoding="UTF-8"?>
    <Configuration status="WARN">
      <Appenders>
        <Lttng name="LTTNG1" domain="LOG4J"/>
        <Lttng name="LTTNG2" domain="LOG4J2"/>
      </Appenders>
      <Loggers>
        <Root level="all">
          <AppenderRef ref="LTTNG1"/>
          <AppenderRef ref="LTTNG2"/>
        </Root>
      </Loggers>
    </Configuration>

Change-Id: I9a41e107d19d5a0efffe055edf0ca5211a7f6d6b
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>