M5: retention policy, prune with repacking, append-only locations

- RetentionPolicy (last/daily/weekly/monthly) and PruneService: forget
  snapshots the policy drops, delete packs nothing uses and repack mostly
  unused ones by copying encrypted blobs as-is; new packs are indexed
  before old ones are removed
- a location lock keeps prune and backups apart; append-only locations
  are never pruned (cleanup via the location's lifecycle rules)
- weekly prune from the background job; occ backup:prune [--dry-run],
  backup:retention, backup:target:append-only
- tests: forget+prune frees space and the remaining snapshot restores
  identically, repack of mixed packs, dry-run preview, retention buckets

lib/Service/BackupService.php

@@ -51,6 +51,9 @@ class BackupService {
 		if ($this->config->getSystemValue('objectstore', null) !== null) {
 			throw new \RuntimeException('Object storage as primary storage is not supported yet');
 		}
+		if ($this->locking->isLocked(PruneService::lockKey($target), ILockingProvider::LOCK_EXCLUSIVE)) {
+			throw new \RuntimeException('A cleanup of ' . $target->getName() . ' is running; try again later');
+		}
 		$running = $this->runs->findRunning($target->getId(), self::KIND_FULL);
 		if ($running !== []) {
 			return $running[0];