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/Backend/IBackend.php

@@ -35,6 +35,17 @@ interface IBackend {
 	/** @return string bytes [$offset, $offset + $length) of the object (used to fetch single blobs from a pack) */
 	public function getRange(string $path, int $offset, int $length): string;
 
+	/**
+	 * Many byte ranges, in the order given (blobs spread over many packs, such as the tree
+	 * segments of a backup). A backend where every request costs a round trip can fetch them
+	 * several at a time.
+	 *
+	 * @param list<array{0:string, 1:int, 2:int}> $ranges [path, offset, length] each
+	 * @return \Generator<int, string> contents by position in $ranges
+	 * @throws BackendException for a range that cannot be read
+	 */
+	public function getRanges(array $ranges): \Generator;
+
 	public function exists(string $path): bool;
 
 	/**