Optimize JUL logging

[romainmoreau]

As stated in http://www.slf4j.org/legacy.html#jul-to-slf4j, the jul-to-slf4j bridge has some performance issues with disabled logging statements.
Since the jul-to-slf4j bridge is activated by default in the spring-boot-starter-logging pom, install a LevelChangePropagator will enhance the performances when the logging system used by the application is JUL and when there are some dependencies using JUL.

[farrault]

In the current default setup, I don't think there is a big performance impact, as only INFO or higher JUL log events are sent to SLF4JBridgeHandler

I created a ticket #2923 to allow the configuration of JUL levels sent to SLF4JBridgeHandler

See also the discussion here http://stackoverflow.com/questions/25364913/spring-boot-set-logging-level-of-external-jar-which-is-using-java-util-logging/30085782#30085782

[philwebb]

See #3924 which solved something similar for Logback. We should still handle other SLF4J implementations.