[JENKINS-20874] lazy load FindBugsMessages singleton

- implemented lazy initialization for FindBugsMessages singleton (using Initialization-on-demand holder idiom)
- I assumed the SaxSetup wrapper in FindBugsPlugin.start() was only needed for the parsing of message xmls
- i only ran "mvn test" to check if things still work (no integration tests)
- due to classloader semantics the synchronization block in FindBugsMessages.initialize() can probably be removed