LOG4J2-631: Update docs to clarify how to use formatter logger and standard logger together

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1609755 13f79535-47bb-0310-9956-ffa450edef68
added performance test for FormatterLogger. Interestingly, FormatterLogger turned out to be faster than {}-style parameters for simple string and double parameters (on Java 8 Windows JVM).

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1609757 13f79535-47bb-0310-9956-ffa450edef68