C++: Lazily computed null_counts counts number of non-null entries

[asfimport]

Reporter: Uwe Korn / @xhochy
Assignee: Uwe Korn / @xhochy

Related issues:

• Arrow release 0.2 (blocks)

_{Note: This issue was originally created as ARROW-543. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy:
PR: #329

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 329
#329