Use plugin classloader when executing task defined in a plugin

[guusdk]

When a clustered tasks is executed (particularly when that task returns a value), class loading issues can occur when the task is defined in a plugin. The classes that are used by the task might not be accessible by the (context class loader of the) thread that executes the task.

When a task is executed that is defined in a plugin, the context class loader of the thread should be switched to the plugin class loader during the execution of the task to work around this issue.

Be aware of #74: Using Tasks defined in a plugin causes issues (particularly when reloading that plugin). That problem remains, even with the improvement suggested here.