Workaround Javadoc bug with JDK 8 (b112+)

Remove Javadoc linkplain to ExceptionHandler#value() from
AnnotationMethodHandlerExceptionResolver to work around JDK
Javadoc bug 9007707.
Polish spring-messsaging

- Removed unused imports
- Organized imports
- Discontinued use of deprecated junit.framework.Assert class
- Suppressed warnings where appropriate
- Added missing generics to return type for getMappingComparator() in
  SimpAnnotationMethodMessageHandler
Polish spring-websocket

- Removed unused imports
- Organized imports
- Deleted unused fields
- Deleted unused local variables
- Deleted unused private static classes
Upgrade to Jetty 9.1.0 .RC0

This commit upgrades Jetty to the latest available 9.1.x version while
also preserving compatibility with 9.0.x.