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

Added `Log4j2Logger` and `Log4j2LoggerProvider`, as well as code to `LoggerProviders` to ensure Log4j 2 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.