[WIP] Prevent the constructor from being called multiple times

[jaepil]

As @hkarel pointed out in #2033, the current implementation of GenericValue::operator[]() calls GenericValue::GenericValue() multiple times when there's no such a member. There's no real issue with that, but it's not ideal. This PR tries to prevent the constructor from being called multiple times.

Note: As I mentioned in #2033, I found some fundamental issues. I will work on it within a few days when I have free time.

[hkarel]

Hello, @jaepil ! Do you have any progress on this PR? When do you plan to include these improvements in the Tencent/rapidjson repo?

[tencent-adm]

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