Qubes: Conversions can sometimes get stuck

[apyrgio]

While working on #574, we have seen that if we introduce some delay in the client (e.g., by converting RGB files to PNG) in the pipeline that reads pixel data, the conversions get stuck. We have seen that the disposable qube is blocked while writing on stdout, and that the client does not make any progress.

NOTE: Currently, we have seen this only if we add online RGB to PNG conversion in the client, when reading the pixel data. Else, the client simply fails the conversion with ENOSPC (see #574). EDIT: This is wrong, turns out that the qube can remain stuck even with the current implementation in main.