Retry on download failure

[georgeliao]

fix #3464

This PR adds binary exponential backoff based retry mechanism to async periodic download tasks, the checking exception (download failure an exception) in the main thread is done in a non-blocking fashion via QFutureWatcher.

1. Changed std::async + std::future pair to QtConcurrent::run + QFuture in order to use some QFuture specific features.
2. Added QFutureWatcher to watch QFuture and react correspondingly in the successful and failed cases when QFuture is finished.
3. Changed DownloadException to a QException-based class so it can cross the qt thread boundary.
4. Made the CRTP BaseQException class to reuse the boilerplate code for all the QException derived classes.

To test the behavior, you can modify the default delay time in daemon.h line 222 to a small number, so the download will run more frequently. After that, the wifi network can be manually turned off in ubuntu settings, which in a way causes download failure.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.00000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 88.83%. Comparing base (2a16af2) to head (42d1612).

Files	Patch %	Lines
include/multipass/async_periodic_download_task.h	85.18%	4 Missing :warning:
include/multipass/exceptions/base_qexception.h	71.42%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3487      +/-   ##
==========================================
+ Coverage   88.82%   88.83%   +0.01%     
==========================================
  Files         254      253       -1     
  Lines       14115    14119       +4     
==========================================
+ Hits        12537    12542       +5     
+ Misses       1578     1577       -1     

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[sharder996]

Hey @georgeliao, thanks for the work on this!

Would you be able to edit the PR description to link the github issues that this fixes in the format 'Fixes #xxxx'? That way I can get some more context on what behavior this is fixing.

Thanks!

[georgeliao]

@sharder996 , sorry that I forgot it, the link is added now. I also added more text in the intro to explain the context. Be free to ask any questions if something is not clear.