Improve org.sonar.api.utils.System2 :

* add to picocontainer
* new method isJavaAtLeast17()
* fix Javadoc
SONAR-4843 disable restart on Windows + Java 6 (UrlClassloader locks JAR files)
Upgrade to plexus-classwords 2.5.1 to unlock JAR files on Windows

UrlClassloader locks JAR files on JVM implementations for Windows. This recent version of plexus-classworlds executes ClassLoader#close() when available (since Java 7)