Advanced monitoring can now be completely disabled, plus some refactoring.

* advanced monitoring will be disabled by default

* moved interface SingleThreadMonitor into subpackage .impl
* moved class WeakChunk into subpackage .impl
* moved subtype ThreadMonitorEvent into its own file in subpackage .impl
* moved subtype SingleThreadMonitorImpl into its own file in subpackage .impl
* renamed class ChunkEvent into ChunkMonitorEvent