Add refresh-exclude-older flag to only transfer new snapshots during instance/volume refresh #2986

Job	Run time
Code (oldstable)	7m 54s
Code (stable)	8m 3s
Code (tip)	10m 38s
Client (oldstable, ubuntu-latest)	27s
System (stable, cluster, dir)	12m 58s
Client (oldstable, macos-latest)	21s
System (stable, cluster, btrfs)	13m 24s
Client (oldstable, windows-latest)	1m 1s
System (stable, cluster, lvm)	14m 54s
System (stable, cluster, zfs)	12m 21s
Client (stable, ubuntu-latest)	31s
System (stable, cluster, ceph)	21m 58s
Client (stable, macos-latest)	30s
System (stable, cluster, random)	13m 4s
Client (stable, windows-latest)	1m 6s
System (stable, standalone, dir)	22m 35s
System (stable, standalone, btrfs)	23m 29s
System (stable, standalone, lvm)	40m 57s
Documentation	5m 6s
System (stable, standalone, zfs)	38m 36s
System (stable, standalone, ceph)	1h 15m 22s
System (stable, standalone, random)	36m 18s
System (oldstable, cluster, dir)	14m 48s
System (oldstable, standalone, dir)	22m 37s
System (tip, cluster, dir)	7m 3s
System (tip, standalone, dir)	25m 12s
	7h 11m 13s

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