Fix: Abort loading log4j agent classes when detected log4j version is too old

The Java Log4j agent rely on the getTimeStamp() method that was
introduced in log4j 1.2.15. This fix implement a runtime detection of
the log4j library capabilities and abort the loading of the relevant
agent classes if the version used is too old.

Thus, log4j tracing will be deactivated on older version of the
library.

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>