Add support for Frozen Objects / Realm's

[leemaguire]

• Add support for Frozen Objects / Realm's
  Usage:

           auto realm = db(std::move(config));
            realm.write([&realm]() {
                realm.add(AllTypesObject());
            });
            auto frozen_realm = realm.freeze();
            MY_ASSERT(frozen_realm.is_frozen();

            auto async = std::async(std::launch::async, [&frozen_realm]() {
                auto thawed = frozen_realm.thaw();
                MY_ASSERT_FALSE(thawed.is_frozen());
                thawed.write([&thawed]() {
                    auto o = AllTypesObject();
                    o._id = 2;
                    thawed.add(std::move(o));
                });
                thawed.write([&thawed]() {
                    auto o = AllTypesObject();
                    o._id = 3;
                    thawed.add(std::move(o));
                });
                MY_ASSERT_FALSE(thawed.refresh());
                MY_ASSERT(thawed.objects<AllTypesObject>().size() == 3);

                MY_ASSERT(frozen_realm.is_frozen());
                MY_ASSERT_FALSE(frozen_realm.refresh());
                MY_ASSERT(frozen_realm.objects<AllTypesObject>().size() == 1);
                return true;
            });
            MY_ASSERT(async.get());

• Add support for sorting experimental::results / managed<std::vector<T>>
  Usage:

            // For sorting on a single property ascending / descending.
            auto sorted_results_ascending = realm.objects<AllTypesObject>().sort("str_col", true);
            // For sorting on a order of multiple properties ascending / descending.
            auto sorted_results_with_descriptors_ascending = realm.objects<AllTypesObject>().sort({{"str_col", true}, {"_id", true}});