Lagging animations with updaters and self.add

Nobody has claimed this yet.

## Description of bug / unexpected behavior

Animations behave differently depending on the way the mobject was added to the scene. If I use `self.play(Create(mobj))` there is no lag, if I use `self.add(mobj)` the animation lags in certain cases.

## Expected behavior

IMHO there should be no difference between the two cases. Also, if using `self.add` is discouraged for objects that have updaters, we might consider printing a warning.

## How to reproduce the issue

See the following code:

Code running with no lagging

```py
from manim import *

class Test(Scene):
def construct(self):
tracker = ValueTracker(-3)
rect = Rectangle().set_x(tracker.get_value())
label = Text("x")
rect.add_updater(lambda m: m.set_x(tracker.get_value()))
label.add_updater(lambda m: m.move_to(rect.get_center()))
self.add(rect,label)
self.play(tracker.animate.set_value(4))
self.wait()
self.play(FadeIn(rect))
self.play(tracker.animate.set_value(-4))
self.wait()
```

Lagging animation in the second part

```py
from manim import *

class Test(Scene):
def construct(self):
tracker = ValueTracker(-3)
rect = Rectangle().set_x(tracker.get_value())
label = Text("x")
rect.add_updater(lambda m: m.set_x(tracker.get_value()))
label.add_updater(lambda m: m.move_to(rect.get_center()))
self.add(rect,label)
self.play(tracker.animate.set_value(4))
self.wait()
#self.play(FadeIn(rect))
self.add(rect)
self.play(tracker.animate.set_value(-4))
self.wait()
```

## Additional media files

https://user-images.githubusercontent.com/52650214/120992430-e36a4300-c782-11eb-80e4-d228dae55858.mp4

Start by running the two provided Test(Scene) reproductions, comparing self.add(rect) with Create(rect) or FadeIn(rect) while the ValueTracker and mobject updaters run. Trace the scene behavior around repeated addition and updater execution. Done means the two cases no longer produce different lag, or the project clearly warns when self.add is unsuitable for objects with updaters.

Written by the indexing model from the issue text.