Patch fastq sync stream

The current implementation of `FastqSync` creates an intermediate Stream that is then passed to a writing function. Apparently, the `Stream` is not evaluated as lazily (or not in the way) as I thought. This led to weird StackOverflow errors (not yet documented).

This fix merges the Stream creation and writing function, so that no intermediate containers are created and no errors are encountered. Tests have been updated accordingly.

See merge request !95