ipn/ipnlocal: use an in-memory TKA store if FS is unavailable

This requires making the internals of LocalBackend a bit more generic,
and implementing the `tka.CompactableChonk` interface for `tka.Mem`.

Signed-off-by: Alex Chan <alexc@tailscale.com>

Updates https://github.com/tailscale/corp/issues/33599

tstest/chonktest/tailchonk_test.go

@@ -39,6 +39,12 @@ func TestImplementsCompactableChonk(t *testing.T) {
 		name     string
 		newChonk func(t *testing.T) tka.CompactableChonk
 	}{
+		{
+			name: "Mem",
+			newChonk: func(t *testing.T) tka.CompactableChonk {
+				return &tka.Mem{}
+			},
+		},
 		{
 			name: "FS",
 			newChonk: func(t *testing.T) tka.CompactableChonk {