Deleting all versions of objects

[rakshil14-2]

Issue :https://github.com/aws/aws-cli/issues/4070

Description of changes: Added a flag for deleting all versions of an object present in a versioned-enable bucket. For dealing with versions, a list-object-versions api is called to get the version ids as well as delete markers of the objects. Once the object and their version ids are retrieved, then a batch of objects with their version id is created which is passed to delete-objects api for deletion.

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