A generic pool class. Simpler and more efficient than goog.structs.Pool because it doesn't maintain a list of objects that are in use. This class has constant overhead and doesn't create any additional objects as part of the pool management after construction time. IMPORTANT: If the objects being pooled are arrays or maps that can have unlimited number of properties, they need to be cleaned before being returned to the pool. Also note that {@see goog.object.clean} actually allocates an array to clean the object passed to it, so simply using this function would defy the purpose of using the pool.