[Go] Optimize memory usage for CSV writer

perhaps not for this PR, but, depending on the number of rows and cols this record contains, this may be a very large allocation, and very big memory chunk.
it could be more interesting performance wise to write n rows instead of everything in one big chunk.

also, to reduce the memory pressure on the GC, we should probably (re)use this slice-of-slice of strings.

**Reporter**: [Anson Qian](https://issues.apache.org/jira/browse/ARROW-4736) / @anson627

_{**Note**: *This issue was originally created as [ARROW-4736](https://issues.apache.org/jira/browse/ARROW-4736). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}