Wait for leader's metadata before starting backups

If an agent on the secondary node starts backup before the leaders one, 
waitForStatus(StatusStarting) could try to read metadata before if was set by the leader. So heartbeat in the meta would be 0 and node assumes that backup stuck (stale heartbeat) and exits with an error.

`waitForStatus` should wait for `pbm.WaitBackupStart` duration for the backup meta