Make thread of Monitor plugin configurable
The attached thread makes the thread class of the Monitor plugin class configurable.
During the migration from CP 3.1 to CP 3.2 we had a major memory leak in our test suite. We start and stop the engine's bus in most of our test closures. Every start leaked a thread and attached objects for 60 seconds (default sleep period for sessions). With the attached thread we can easily use a PerpetualTimer instead of a BackgroundTask for all our monitors during test runs.