Previously, Log4J2LoggingSystem did not install SLF4JBridgeHandler.
SLF4JBridgeHandler is used to route logging calls made using
java.util.logging into SLF4J. This resulted in entries being written
to System.err instead.

This commit updates Log4J2LoggingSystem to register the bridge handler
thereby ensuring the logging that originates into java.util.logging is
routed into SLF4J. This change has been made by extracting a new
abstract superclass that performs the registration. This super class is
extended by all SLF4J-based LoggingSystem implementations.

Fixes gh-1737