[C++] Add Flush method to arrow::io::OutputStream

[asfimport]

Some output sinks have a flush() method separate from write(...). Rather than flushing all writes, we can leave this up to the implementation, and have a default no-op Flush implementation. This came up during ARROW-404 (ran into test failure with unflushed writes, because the Java HDFS client does not auto-flush)

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• [Python] Closing an HdfsClient while there are still open file handles results in a crash (relates to)

_{Note: This issue was originally created as ARROW-410. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
PR: #310

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 310
#310