This change now flushes all pending messages from the backendmanager to the database, even if the main operation crashes (or is stopped).
This ensures that the database and remote storage are more in sync.
Also removed a redundant warning from the backendmanager during shutdown, and simplified a check for active operations.
This fixes#6200
While investigating issue [#6103](https://github.com/duplicati/duplicati/issues/6103), found that connections were not disposed due to the backend pool in BackendManager's handler. Set up a FileZilla server to test the issue before and after the change, confirming that connections now close as expected with this fix.
* Rewrite the backend manager.
This creates a new more logic backend manager that handles all backend operations.
For each top-level operation, there is now a single backend manager instance that is passed to sub-commands.
The internals of the backend manager are now rewritten to use async logic instead of threading.
The design uses a single task runner that dispatches operations and honors concurrent settings and limits from one place.
The logic for each operation has been moved into a separate files/classes to make it easier to understand each operation.
This fixes#5804
* Fixed a few issues with not awaiting tasks
* Fixed not creating empty databases
* Fixed an issue with hashes not being recorded on download
* Reworked the download logic to avoid attempting to decrypt the file if it has been modified.
* Review fixes
* Renamed db collector to better reflect the purpose.
* Review fixes
