Fast S3 verify and snapshot commit; shorter runs list; durations with days

On S3, index files and the tree segments read back when a snapshot is
written are fetched 16 at a time through the storage's S3 client
(IBackend::getRanges(), getMany()). Blobs that follow each other in a pack
are read as one range on every location. The snapshot is written in a step
of its own and the run shows "commit" meanwhile. On a 1 TB production
repository the check went from about 4 hours to 3.5 minutes.

backup:status and the web interface list the runs of the last 7 days,
running ones and at least the newest 5; --all / "Show older runs" list
all. Durations of a day or more show the days.

lib/Repository/PackReaderGuard.php

@@ -38,6 +38,11 @@ final class PackReaderGuard implements IBackend {
 		return $this->inner->getRange($path, $offset, $length);
 	}
 
+	public function getRanges(array $ranges): \Generator {
+		$this->reader->close();
+		return $this->inner->getRanges($ranges);
+	}
+
 	public function exists(string $path): bool {
 		$this->reader->close();
 		return $this->inner->exists($path);