Add a Log4j 2.x agent specific domain 'log4j2'

Add a Log4j 2.x agent specific domain 'log4j2'

The initial version of the new LTTng-UST Log4j 2.x agent only operated
in a compatibility mode making use of the existing 'log4j' tracing
domain currently implemented in LTTng-Tools.

While this is useful when migrating existing Log4j applications using
the compatibility bridge it does require converting the log levels from
the new Log4j 2.x values to the old Log4j 1.x standard. This results in
hiding the actual log level values from the users for applications
natively using Log4j 2.x.

Exposing the native Log4j 2.x log level values requires a new domain
since the changes are significant:

  * The same list of standard log levels and names
  * Each standard log level has a new integer value
  * The log levels scale is reversed and shortened from
    'int32_max -> int32_min' to '0 -> int32_max'
  * The interval between standard log levels has changed

This new 'log4j2' domain is basicaly a straight copy of the current
'log4j' domain with minor adjustements for the reversed and shortened
scale.

Change-Id: I89f9c0a428ffe1d0bd26f7af547e9e21503de653
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>