Remove writeRecordsCsv, refactor the usage metrics task for Redshift

[misolt]

• remove writeRecordsCsv which worked on lists of a file's all records
• create CsvRecordWriter for writing records one at a time
• replace Optional with Nullable to simplify a helper method
• move the list of extracted cluster usage metric types to RedshiftClusterUsageMetricsTask
• refactor record formatting in RedshiftClusterUsageMetricsTask

We use a list of types of metrics in this connector. The refactor of the usage metrics task relies on that list being a constant. After merging this change, we still can introduce more metric types, but creating custom types of metrics dynamically would require a different approach.