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/Repository/PackWriter.php

@@ -59,7 +59,8 @@ final class PackWriter {
 		fwrite($this->buffer, $header . pack('N', strlen($header)));
 		rewind($this->buffer);
 		$path = 'packs/' . substr($packId, 0, 2) . '/' . $packId;
-		$this->backend->put($path, $this->buffer);
+		$this->backend->put($path, $this->buffer); // closes the buffer
+		unset($this->buffer);
 		$this->index->addPack($packId, $this->entries);
 		$this->reset();
 	}