DRILL-3920 Add vector loading tests

[cwestin]

Additional tests added to TestValueVectors for serialization and loading; these tests were used to find and debug issues with DrillBuf slicing. Some light cleanup of a few vector implementations.

Unit tests pass.
Regression suite has a few (known recurring) failures:

• timeouts while waiting to send intermediate work fragments
• timeouts for "Fetch parquet metadata" tasks not complete
• Selected column 'dir0' must have name 'columns' or must be plain '*'

[jacques-n]

Can you break each of these commits into its own jira? It is confusing that you are doing a bunch of independent commits under the same jira number.

[cwestin]

This is now https://issues.apache.org/jira/browse/DRILL-3920 .

[hnfgns]

the logic in clear() seems duplicating close(). we should directly call close perhaps.

[cwestin]

But clear() doesn't call vectors.clear(). I seem to recall a problem with adding that to clear() in the past. I'm worried this will introduce a regression, so I'd rather not take that change on now.

[cwestin]

I made the changes indicated above, but not any that might destabilize tests (noted above). Please re-review and merge.

[hnfgns]

+1