public final T alloc() {
        T e = pool.poll();
        return e != null ? e : factory.get();
    }
Allocates an instance of the resource to the requestor.
The resource will be allocated exclusively to the requestor until it calls free(instance).

@return a reused or newly created instance of the resource
vespajlib/src/main/java/com/yahoo/yolean/concurrent/ConcurrentResourcePool.java