[Spike] LB3-style relations

[bajtos]

This is a follow-up for #2224.

After #2224 is done and we understand how to build a a general mechanism for dropping LB3 models into LB4 apps, we need to take a look on LB3-style relations.

• LB4 needs to process definition of relations provided in LB3 (LDL) metadata and via JS API like MyModel.belongsTo. These relations should be setup via the existing implementation provided by juggler.
• Remote methods contributed by relations must be exposed in REST API of the target LB4 app.

[b-admike]

From Raymond: focus should be more on the JSON (declarative) relation metadata support instead of using JS APIs.

[bajtos]

We decided to stop pursuing this direction, see #2224 (comment)