SFTP: replace a dropped connection; write the index cache in one transaction

A Hetzner Storage Box drops an SFTP connection that carried a lot of data
and then stayed quiet for a few minutes; files_external keeps using the dead
client, on which every file looks missing. StorageBackend now checks the
connection after a minute without requests, and replaces a connection that
is dead after a failed request (once per request; put, move and delete are
not repeated). SftpReadStream closes a connection it left in an unknown state.

The verify cache refresh took more than 6 minutes on PostgreSQL for 27,547
packs, one autocommitted insert per pack: putMany()/replaceAll() write in one
transaction. After a verify the cache holds the index files as they are, so a
blob in two packs stays cached for both.

lib/Service/TargetService.php

@@ -17,6 +17,7 @@ use OCA\NgBackup\Db\TargetMapper;
 use OCA\NgBackup\Repository\Repository;
 use OCP\App\IAppManager;
 use OCP\AppFramework\Db\DoesNotExistException;
+use OCP\Files\Storage\IStorage;
 use OCP\Security\ICrypto;
 use OCP\Server;
 
@@ -279,7 +280,8 @@ final class TargetService {
 	}
 
 	private function buildBackend(string $backend, string $auth, array $options, string $basePath, bool $createBase = true): StorageBackend {
-		return new StorageBackend($this->factory()->create($backend, $auth, $options), $basePath, $createBase);
+		$create = fn (): IStorage => $this->factory()->create($backend, $auth, $options);
+		return new StorageBackend($create(), $basePath, $createBase, $create);
 	}
 
 	private ?ExternalStorageFactory $factory = null;