[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.*}

Start with the vector branch linked in the issue and inspect the ColumnVector implementation to trace how its length is calculated. Confirm that length is reported in rows rather than DataPages, then use PR-257 and the existing test coverage to verify the expected behavior.

Written by the indexing model from the issue text.