Merge pull request #62 from jeremeamia/feature-throttling-retries

Handle throttling for all services.
Expose the 'gc_operation_delay' config value for the DynamoDB Session Handler
Added 509s to the retry list and improved backoff-related comments
Added a guide for the DynamoDB Session Handler
Made minor updates to DynamoDB Session Handler doc
Added content about the new gc_operation_delay option to the DynamoDB Session Handler guide