Migration to log4j 2

We should get rid of log4j 1.2.x dependency in resteasy-core/resteasy-core-spi and move to log4j 2. In order to do so, besides fixing maven dependencies, xml configurations of log4j have to be converted and the org.jboss.resteasy.test.resteasy_jaxrs.i18n.TestLogMessages_Abstract test has to be re-written.
Relying on the 1.2 api bridge (log4j-1.2-api-2.x) does not seem to be a viable solution, as the above mentioned test relies on log4j 1.2 internals.

Some references:

• https://logging.apache.org/log4j/log4j-2.8/manual/migration.html
• https://logging.apache.org/log4j/2.0/faq.html