Fix `RangeResult.readThrough` misuses:
1. KeyValueStores do not need to set readThrough, as it will not be
serialized and return. Also setting it to the last key of the result
is not right, it should at least be the keyAfter of the last key;
2. Fix NativeAPI doesn't set `RangeResult.more` in a few places;
3. Avoid `tryGetRange()` setting `readThrough` when `more` is false,
which was a workaround for the above item 2;
4. `tryGetRangeFromBlob()` doesn't set `more` but set `readThrough` to
indicate it is end, which was following the same above workaround I
think. Fixed that.
5. `getRangeStream()` is going to set `more` to true and then let the
`readThrough` be it's boundary.
Also added readThrough getter/setter function to validate it's usage.
Jay Zhuang