ConstNode GetAsXXXX methods refactor

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This issue is going to to Refactor ConstNode's GetAsXXXX methods, e.g. `GetAsString()`, `GetAsDouble` etc, since it is hard to know if these GetAsXXXX methods go well or not. So we have to re-design the methods as follows:
```
bool GetAsInt(int* output);
bool GetAsString(std::string* str)
```

Also, we will support GetAsString for types like `kBool`, `kDate`, `kTimestamp`.
1. kBool: `true` -> `"true"`, kBool: `false` -> `"false"`
2. kTimestamp: `1590115420000` -> `"1590115420000L"`
3. kDate:
```
Date date(2020, 05, 22);
to_string(date.date_)
```
need to discuss what `kDate` string looks like. `yyyy-mm-dd` pattern string will be far more reasonable.

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