Introduce cascading of CERs (incl. propagating of revisions)

Hi,

based on the issue https://www.drupal.org/node/2885099 regarding revision support of references I would like to bring the idea of composite entities https://www.drupal.org/node/2822476 to your attention.

The basic idea of a composite entity is that it comprises multiple entities, their references on revision basis (instead of on node basis).
cer could prove the central element for that idea: by means of a configurable hierarchy or cascade of CERs.

Now cer allows for updating one reference on two entities:

A <-> B

A cascade could like this:

C <-> D
      D <-> E

The basic purpose of it would be to propagate a coherent state of revisions from C -> E and E->C, depending whether C or E were changed.

Examples

In the example I take revision support of cer as given
Think of this situation showing the node revisions:

F (Rev 1) <-> G (Rev 1) 
              G (Rev 1)  <-> H (Rev 1)

Scenario a)

Changing a value on F - without a revision propagating cascade
-> changes the revision of F and updates the reference field of G, creating a new revision of G (since the reference on G must change it also causes a new revision).
Without a propagating cascade the reference pointing to H stays untouched. We'd have:

F (Rev 2) <-> G (Rev 2) 
              G (Rev 1)  <-> H (Rev 1)

Next change a value on G and you get:

F (Rev 3) <-> G (Rev 3) 
              G (Rev 3)  <-> H (Rev 2)

Next change a value on H and you get:

F (Rev 3) <-> G (Rev 3) 
              G (Rev 4)  <-> H (Rev 3)

Looking at it on a node basis it is correct. But if you want to go back in time you have lost the historical connections. At least I don't know a way of how to present the changes over the cascade.

Scenario b)

Changing a value on F - with a revision propagating cascade same steps as above, but the revisions get all updated if one of the cascaded entities gets a new revision:

- change the revision of F

F (Rev 2) <-> G (Rev 2) 
              G (Rev 2)  <-> H (Rev 2)

Next change a value on G and you get:

F (Rev 3) <-> G (Rev 3) 
              G (Rev 3)  <-> H (Rev 3)

Next change a value on H and you get:

F (Rev 4) <-> G (Rev 4) 
              G (Rev 4)  <-> H (Rev 4)

While the Revision Ids may of course not be identical (you may have connected a new entity to one that already exists with a number of revisions) you get the idea.

Whenever you want to "travel back in time" to see what was entered in a document and what it was referencing at that time is now possible - with a cascade that propagates revsions.

I have explained the importance of this mechanism in the complex composite entities issue.
cer is closest to this functionality - of all modules I know.

If cer would support revisions and configurable propagating hierarchies/cascades ... it would make drupal soo much more powerful than it is today.

P.S.: thank you for cer! it already makes life soo much better!