Memory connector should enforce referential integrity

[bajtos]

The memory connector should enforce foreign key and uniqueness constraints for HasMany, HasOne and BelongsTo relations.

For example, when creating an order using /post/{userId}/orders for a userId which is not that of a user, I would've expected some sort of a check that would've ensure that the userId exists. The connector should throw an error for an userId not in the database.

See #1718 for background and #2332 for the SQL story.

See https://github.com/strongloop/loopback-connector-rest/issues/134 for a Connector helper to parse different styles of index/fk definitions into a format that's easy to consume by connectors.

Acceptance criteria

• Modify the built-in memory connector to enforce unique indexes defined at model and property level, leverage the new helper implemented by https://github.com/strongloop/loopback-connector-rest/issues/134. See Sugar response decorators for easier controller config #1672 for the description of different ways on how model indexes can be defined. See feat: add unique index support on create for in-memory connector loopback-datasource-juggler#1672 for prior implementation that was not finished.
• Modify the buit-in memory connector to enforce FOREIGN KEY constraints defined at model and property level, leverage the new helper implemented by https://github.com/strongloop/loopback-connector-rest/issues/134. See Sugar response decorators for easier controller config #1672 for the description of different ways on how model indexes can be defined.
• Warn the user about index/foreign-key options that are not supported by memory connector (see out-of-scope below).

Out of scope

• "Plain" indexes without uniqueness check
• onUpdate and onDelete options for foreign keys

[bajtos]

The pull request loopbackio/loopback-datasource-juggler#1672 contains an early version of UNIQUE index support for our memory connector.

[jannyHou]

@strongloop/loopback-next Let's add the acceptance criteria for this story.

[bajtos]

I think we need to work on #2606 first and the implement any follow-up stories.

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