fs/cache: make sure we call the Shutdown method on backends

This change ensures we call the Shutdown method on backends when
they drop out of the fs/cache and at program exit.

Some backends implement the optional fs.Shutdowner interface. Until now,
Shutdown is only checked and called, when a backend is wrapped (e.g.
crypt, compress, ...).

To have a general way to perform operations at the end of the backend
lifecycle with proper error handling, we can call Shutdown at cache
clear time.

We add a finalize hook to the cache which will be called when values
drop out of the cache.

Previous discussion: https://forum.rclone.org/t/31336

lib/cache/cache_test.go

@@ -333,3 +333,37 @@ func TestCacheRename(t *testing.T) {
 
 	assert.Equal(t, 1, c.Entries())
 }
+
+func TestCacheFinalize(t *testing.T) {
+	c := New()
+	numCalled := 0
+	c.SetFinalizer(func(v interface{}) {
+		numCalled++
+	})
+	create := func(path string) (interface{}, bool, error) {
+		return path, true, nil
+	}
+	_, _ = c.Get("ok", create)
+	assert.Equal(t, 0, numCalled)
+	c.Clear()
+	assert.Equal(t, 1, numCalled)
+
+	_, _ = c.Get("ok", create)
+	c.Delete("ok")
+	assert.Equal(t, 2, numCalled)
+
+	_, _ = c.Get("ok", create)
+	c.DeletePrefix("ok")
+	assert.Equal(t, 3, numCalled)
+
+	_, _ = c.Get("old", create)
+	_, _ = c.Get("new", create)
+	c.Rename("old", "new")
+	assert.Equal(t, 4, numCalled)
+
+	c.expireDuration = 1 * time.Millisecond
+	_, _ = c.Get("ok", create)
+	time.Sleep(2 * time.Millisecond)
+	c.cacheExpire() // "ok" and "new" fall out of cache
+	assert.Equal(t, 6, numCalled)
+}