Allow to find out when the pool is completely filled

[radarfox]

There is no way to tell, when the filling of minimum objects is completed. Or am I missing something? I would suggest:

• at least emit an event, that the pool is ready for use,
• or better add the fill method which would return Promise resolved on completion,
• or best add the fill method with count parameter.

[radarfox]

• or make the start method return Promise.