QoS handles for fetch_async requests

[S-S-X]

For fetch_async requests handle_index will never reset / reuse released handles.

This is not really that big problem in practice because most probably server will not survive long enough to get into problems because of bad-precision lua nuimbers and things like that.
For most servers there's high probability that sun will explode before counter gets high enough.

This however is still bug and should be fixed but with low priority and without any performance impact, so something simple and efficient should be used if fixing this one.