Make it easier to reliably test data observers

In my experience, testing data observers is somewhat brittle. Even with explicit transactions I sometimes get "delayed" notifications that have been triggered by the previous operation.

Let's take this test (ObjectBox 2.2.0 + RxJava 2):

boxStore.runInTx {
    boxStore.boxFor(SomeEntity::class.java).put(SomeEntity())
}

val testObserver = TestObserver.create<Optional<SomeEntity>>()

boxStore.runInTx {
    repository.find(123).subscribeWith(testObserver)
}

testObserver.awaitCount(1)
testObserver.assertValueCount(1)

Sometimes, I get a value count of 2 from the original put operation. For completeness, the code of the find() method:

fun find(id: Long): Observable<Optional<SomeEntity>> {
    return Observable.create { emitter ->
        val query = this.boxStore.boxFor(SomeEntity::class)
            .query()
            .equal(SomeEntity_.id, id)
            .build()

        query.subscribe().observer { result ->
            emitter.onNext(result.firstOrNull().toOptional())
        }
    }
}

Assuming I did not misunderstand the docs or misuse the ObjectBox API, it would be great if ObjectBox could offer some method e.g. to block until all notifications triggered by the last transaction have been issued.

Comece com a sequência runInTx, TestObserver e query.subscribe().observer mostrada na issue e, em seguida, analise o comportamento de observação de dados e de transações do ObjectBox que ela exercita. O trabalho estará concluído quando houver uma forma confiável de aguardar as notificações da transação anterior, demonstrada por um teste que receba consistentemente o único valor esperado.