perf: Delete previews in one go instead of individually

[artonge]

With the new preview backend, we are deleting preview individually, this is probably expensive and slow.

server/lib/private/Preview/PreviewService.php

Lines 112 to 132 in b6e90ac

	public function deleteAll(): void {
	$lastId = 0;
	while (true) {
	$previews = $this->previewMapper->getPreviews($lastId, 1000);
	$i = 0;
	// FIXME: Should we use transaction here? Du to the I/O created when
	// deleting the previews from the storage, which might be on a network
	// This might take a non trivial amount of time where the DB is locked.
	foreach ($previews as $preview) {
	$this->deletePreview($preview);
	$i++;
	$lastId = $preview->getId();
	}
	if ($i !== 1000) {
	break;
	}
	}
	}

Shouldn't we do it in one go?

1. Flush the preview table
2. Delete the preview directory

Previously it was done individually too.