Fix flushing rows in the end, correctly wait for drain event, restructure streams.

[jirimoravcik]

This PR fixes an issue, where in some cases, the row transform stream would not transform all rows and the XLSX file would be incomplete. Also can be considered a refactor since the code got simplified (no need to use a PassThrough stream anymore).

• Remove PassThrough stream and add sheet header and footer directly in the RowTransform class
• Correctly wait for drain event when pushing to RowTransform to prevent memory buffering. (based off Node.js docs https://nodejs.org/api/stream.html#stream_writable_write_chunk_encoding_callback)
• Update unit tests to reflect the new RowTransform behavior and add a larger high level test (currently a file with 500 rows - that's a size on which I was able to observe the flushing issue)

[mtrunkat]

that's a bit of magic :)