[JBLOGGING-95] Add first-class support for Log4j 2

Added Log4j2Logger and Log4j2LoggerProvider, as well as code to
LoggerProviders to ensure Log4j2 is used if on the classpath. Also
resolved about a dozen compiler warnings in Logger due to methods
being @deprecated in Javadoc but not @Deprecated in code.
[JBLOGGING-95] MDC map values may be null; should use HashMap
[JBLOGGING-95] Making darn sure we have a valid Log4j 2 API