URLClassLoader leak in PluginLoader

[gtoison]

PluginLoader creates some URLClassLoader and never closes them. When the URLClassLoader points to a .jar file, the JVM process keeps a reference to that file (a RandomAccessFile instance) preventing from deleting/renaming that .jar file.
This is an issue when the JVM process is long lived, for instance when it's running inside the Gradle daemon (see spotbugs/sonar-findbugs#128)

The RandomAccessFile is created as soon as the classloader opens the .jar file, which for me was when looking for the non-existing resource messages_en_GB.xml. The URL does not exist because the .jar does not contain it so calling JarURLConnection.setUseCaches() wouldn't help here.

To sum up, this code reproduces the issue: omitting the close() on the last line creates a lock on the plugin jar file until the JVM process is stopped.

URLClassLoader classLoader = new URLClassLoader(new URL[] {workingPluginFile.toURI().toURL()});
classLoader.findResource("messages_en_GB.xml");
classLoader.close();

[gtoison]

This can be closed now that #2025 is merged