Avoid dynamic dispatching when unwrapping Hive data.

This is a follow up of PR #758.

The `unwrapHiveData` function is now composed statically before actual rows are scanned according to the field object inspector to avoid dynamic dispatching cost.

According to the same micro benchmark used in PR #758, this simple change brings slight performance boost: 2.5% for CSV table and 1% for RCFile table.

```
Optimized version:

CSV: 6870 ms, RCFile: 5687 ms
CSV: 6832 ms, RCFile: 5800 ms
CSV: 6822 ms, RCFile: 5679 ms
CSV: 6704 ms, RCFile: 5758 ms
CSV: 6819 ms, RCFile: 5725 ms

Original version:

CSV: 7042 ms, RCFile: 5667 ms
CSV: 6883 ms, RCFile: 5703 ms
CSV: 7115 ms, RCFile: 5665 ms
CSV: 7020 ms, RCFile: 5981 ms
CSV: 6871 ms, RCFile: 5906 ms
```

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #935 from liancheng/staticUnwrapping and squashes the following commits:

c49c70c [Cheng Lian] Avoid dynamic dispatching when unwrapping Hive data.