Add non-blocking pop for avalon streams.

[tasgomes]

The Avalon stream slave component does not support non-blocking 'pop_avalon_stream()' while the AXI stream slave component does by using 'pop_axi_stream()' together with 'await_pop_axi_stream_reply()'.

This updates the Avalon stream slave with a non-blocking 'pop_avalon_stream()'.