Introduce our own logging abstraction, used throughout. Provide implementations of this abstraction and factory methods that default to the slf4j implementation but can use a java.util.logging variant, switched via a system property. Remove finer/finest j.u.l-specific levels, change these usages to fine (with comments added at each usage, should these need changing/reinstating); retain config as a 'level' but log it at debug level. Default 'checked' level is now info. Other than SLF4JLogger, only HttpMessageReceiver uses j.u.l directly as Jetty's logging is coupled to it.
Rename 'fine' logging methods to 'debug'.
Rename 'warning' methods to 'warn'.
Rename 'severe' methods to 'error'.