Add support for Flexible Sync

[cmelchior]

Working API:

abstract class BaseRealm {
    @ObjectServer
    @Beta
    public SubscriptionSet getSubscriptions() { ... }
}

@ObjectServer
@Beta
public interface Subscription {

    static Subscription create(String name, RealmQuery query) { ... }
    static Subscription create(RealmQuery query) { ... }

    @Nullable Date getCreatedAt();
    @Nullable Date getUpdatedAt();
    @Nullable String getName();
    String getObjectType();
    public String getQuery();
}

@ObjectServer
@Beta
public interface SubscriptionSet extends Iterable<Subscription> {

    public enum State {
        UNCOMMITTED(OsSubscriptionSet.STATE_VALUE_UNCOMMITTED),
        PENDING(OsSubscriptionSet.STATE_VALUE_PENDING),
        BOOTSTRAPPING(OsSubscriptionSet.STATE_VALUE_BOOTSTRAPPING),
        COMPLETE(OsSubscriptionSet.STATE_VALUE_COMPLETE),
        ERROR(OsSubscriptionSet.STATE_VALUE_ERROR),
        SUPERCEDED(OsSubscriptionSet.STATE_VALUE_SUPERCEDED);

        public static State fromNativeValue(long value) { ... }
    }

    @Nullable Subscription find(RealmQuery query);
    @Nullable Subscription findByName(String name);
    State getState();
    int size();
    @Nullable String getErrorMessage();
    boolean waitForSynchronization();
    boolean waitForSynchronization(Long timeOut, TimeUnit unit);
    RealmAsyncTask waitForSynchronizationAsync(Callback callback);
    RealmAsyncTask waitForSynchronizationAsync(Long timeOut, TimeUnit unit, Callback callback);
    SubscriptionSet update(UpdateCallback action);
    RealmAsyncTask updateAsync(UpdateAsyncCallback callback);
    void refresh();

    interface UpdateCallback {
        void update(MutableSubscriptionSet subscriptions);
    }

    interface UpdateAsyncCallback extends UpdateCallback {
        void onSuccess(SubscriptionSet subscriptions);
        void onError(Throwable exception);
    }

    interface Callback {
        void onStateChange(SubscriptionSet subscriptions);
        void onError(Throwable e);
    }
}

@ObjectServer
@Beta
public interface MutableSubscriptionSet extends SubscriptionSet {
    Subscription add(Subscription subscription);
    Subscription addOrUpdate(Subscription subscription);
    boolean remove(Subscription subscription);
    boolean remove(String name);
    boolean removeAll(String objectType);
    <T extends RealmModel> boolean removeAll(Class<T> clazz);
    boolean removeAll();
}

[cmelchior]

I should have addressed all comments. Please take another look.