Add Worker/EngineClient autoclose option

[thiell]

This ticket will follow the addition of "autoclose" clients or timers (both derives from EngineBaseTimer), allowing task.resume() to exit even when these clients/timers are not finished.

In depth, currently registered Engine clients in ClusterShell are counted at each loop by the runloop (called by task.resume()), that means the runloop will not exit until all engine clients are finished or explicitely terminated. We will modify this behavior by adding a runloop reference counter, and "autoclose" clients will not take a refcount.

[thiell]

[st-cea] Autoclose support committed in revision 92.