monitors are always inheritable
[MSC-122] another unstableServices >= 0 invariant fixes
 * update unstableServices counter only if monitor is really added or removed from observers list
 * refactoring StabilityMonitor.remove() -> StabilityMonitor.removeController(Controller)
introducing two additional awaitStability methods + reordering StabilityMonitor methods to be paired properly
introducing StabilityStatistics
fixing StabilityMonitor deadlocks
enrich test case to use StabilityStatistics
providing StabilityMonitor & StabilityStatistics javadoc