Implement Key Generation Strategy

[tristanlins]

The current implementation only generates IDs for NULL values.
But in JPA or in particular Hibernate generates a new ID for all unmanaged entities (If an ID generator is used).

In Kotlin in particular, this can be exploited as follows:

@Entity
data class Media(
  @Id
  @Column(nullable = false)
  @GeneratedValue
  var id: UUID = UUID(0, 0)
)

The "id" field is initialized with a 0-UUID and is never NULL.
This is a popular approach, mainly because of the NULL safety in Kotlin.
In order not to change the previous behavior, there is now a KeyGenerationStrategy with which you can change the behavior:

val repository = RepositoryFactoryBuilder
    .builder()
    .generateKeysUsing(UUIDObjectTypeKeyGenerator::class.java, KeyGenerationStrategy.ALL_UNMANAGED)
    .mock(MyRepository::class.java)

An entity that is not yet contained in the DataStore is seen as "unmanaged".

---

Besides, the mock is not really complete.
When an "unmanaged" entity is saved, Hibernate usually creates a clone.
This is therefore useful in order to prevent side effects from object manipulation.
The mock does not do that, which for example leads to the following code not working (in JPA + Hibernate, however, it does):

class Controller {
  fun userDetails(id: UUID): User {
    val user = repository.findByIdOrNull(id)
    user.eraseCredentials() // Manipulation of the object also manipulates the DataStore
    return user
  }
}

However, a (serialized) clone should perhaps be saved in the DataStore in order to prevent manipulation in the DataStore?

[coveralls]

Coverage decreased (-0.2%) to 98.698% when pulling 5ae838e on tristanlins:feature-key-generation-unmanaged-entities into e854772 on mmnaseri:development.

[mmnaseri]

Thanks a lot for your PR! This is great code. A few very minor comments left for you. Also, would you mind running your code through google-java-format? If you don't have it installed, that's alright, I can do it when merging to master. If you can, it would also be great to have this change explained in either the README (same PR) or the gh-pages (separate PR)

[tristanlins]

The desired changes have been made. 👋
See #187 for documentation update.