[pgAdmin4][patch][runtime]: RM#2829, RM#2491 - pgAdmin4 crashes while saving CSV data from Query tool

Hi,

I am able to reproduce the crash while downloading and save data in CSV
file from query tool.

Please find attached updated patch with below changes after reading Qt
documentation.

- Added new signal "readyRead". As per the Qt documentation, this signal
will be emitted when data is ready from IO channel for large amount of data
transfer between server and client.
- Ready read and DownloadInProgress signal is very quick in call so we
should not do large operation inside that slot because for downloading big
data it may possible of frequent call of those signals which may cause the
crash or missing data to write inside the file so removed unnecessary logic
from that slot.
- Fixed the crash while opening IODevice with NULL handle.

With above changes, I have tested with same data as earlier and it is
working as expected without crashing the application.

Do review it and let me know for comments.

Thanks,
Neel Patel