ArangoDB: use _key instead of _id

[rashtao]

TODO: rebase onto #292 once merged

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In ArangoDB, the field _key should be considered as primary key for the documents. In fact, _id field is read-only (the database ignores the values set by the client) and auto-generated by the database (concatenating <collection-name>/<_key>.

See https://docs.arangodb.com/stable/concepts/data-structure/documents/#document-keys

For the moment, I have annotated the id fields in the test entities specifying _key value, e.g.:

@Entity
public class Book {

    @Id("_key")
    private String id;

    // ...
} 

Ideally in future we could allow databases implementation specify the default field name, i.e. allowing:

@Entity
public class Book {

    @Id
    private String id;

    // ...
} 

to be mapped automatically to field _key when using ArangoDB.

[genie-jnosql]

Can one of the admins verify this patch?