* 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
* Simplified task control by having all logic in a single class and using async control mechanism
* Fixed the tests
* Made the marking of a backup partial explicitly communicated if the enumeration process is stopped.
* Added support for pausing active transfers
* Introduced transfer token to abort operations.
* Updated the way LiveControl is emitting events.
* Retains the paused state of the server across reboots, this fixes#5760.
* Reworked the way throttle speeds are propagated through LiveControls.
* Added option to pause transfers in the UI.
* Removed throttle settings from the LiveControls class so it is exclusively handled by the ApplicationSettings.
* Removed `thread-priority` as it does not work at all when tasks are used.
* Fixed the stop dialog and API to only support stop and abort.
* Removed unused variables
