* 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
* Added test to verify that compression modules will mask duplicate entries, if present
* Because there are likely many, otherwise valid, dindex zip archives with duplicated blocklists out there, the log message is now verbose.
To make sure we no longer create archives with duplicated entries, the `--unittest-mode` option makes this a warning that is checked in most tests.
* Added guard in `IndexVolumeCreator` to prevent creating zip files with duplicated block lists.
* Added debug check for duplicates
* Fixed the duplicate test
Encrypting, hashing, and creating index volumes are now done
in the DataBlockProcessor and the SpillCollectorProcess. This allows
uploads to always be transferring data and not have to stop to create
a file, encrypt data, etc.
