I added a RemoveAll() method on tka.Chonk in #17946, but it's only used
in the node to purge local AUMs. We don't need it in the SQLite storage,
which currently implements tka.Chonk, so move it to CompactableChonk
instead.
Also add some automated tests, as a safety net.
Updates tailscale/corp#33599
Change-Id: I54de9ccf1d6a3d29b36a94eccb0ebd235acd4ebc
Signed-off-by: Alex Chan <alexc@tailscale.com>
85373ef822