[Vectorized Reader] ColumnVector length should be in terms of rows, not DataPages

In https://github.com/zhenxiao/incubator-parquet-mr/tree/vector
ColumnVector length is in terms of DataPages, need to be in terms of rows

**Reporter**: [Zhenxiao Luo](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=zhenxiao) / @zhenxiao
**Assignee**: [Nezih Yigitbasi](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=nezihyigitbasi) / @nezihyigitbasi
#### PRs and other links:
- [PR-257](https://github.com/apache/parquet-mr/pull/257)

_{**Note**: *This issue was originally created as [PARQUET-299](https://issues.apache.org/jira/browse/PARQUET-299). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

从 issue 中链接的 vector 分支开始，检查 ColumnVector 的实现，以追踪其 length 的计算方式。确认 length 是按行而不是按 DataPages 报告的，然后使用 PR-257 和现有的测试覆盖率来验证预期行为。

由索引模型根据 Issue 内容生成。