Phase 0: repository on files_external storages without a mount (spike D)

ExternalStorageFactory builds storages with files_external's own
backends and auth mechanisms from an unsaved StorageConfig (as its
ConfigAdapter does), and StorageBackend adapts any IStorage to IBackend
(writeStream, seekable range reads, temp name + rename except on S3).
IBackend::put() now owns and always closes the stream, because
files_external's S3 writeStream closes it.

Spike D (NC34): S3 (SeaweedFS) and SFTP: 120 MiB backup ~6s, unchanged
run uploads nothing, restore bit-identical, no files_external mount
created, no temp files (max 12 KiB in /tmp). Works with files_external
disabled when its classes are loaded. Backup peak memory 85-89 MiB on
these targets (to investigate).

lib/Backend/LocalBackend.php

@@ -35,6 +35,9 @@ final class LocalBackend implements IBackend {
 			}
 		} finally {
 			fclose($out);
+			if (is_resource($stream)) {
+				fclose($stream);
+			}
 		}
 		if (!rename($tmp, $target)) {
 			@unlink($tmp);