* 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
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.
Some changes were required for this merge,
now the StreamBlockSplitter process will take in any stream, store the blocks and return the blockset ID.
This makes it compatible with the changes for metadata handling where they support more than a single block of metadata.
# Conflicts:
# Duplicati/Library/Main/Database/LocalDatabase.cs
# Duplicati/Library/Main/Database/LocalRepairDatabase.cs
# Duplicati/Library/Main/Operation/BackupHandler.cs
# Duplicati/Library/Main/Operation/RecreateDatabaseHandler.cs
