Cleanup code and make it consistent across all handlers

[dsanders11]

Issue #, if available:

Description of changes:

The current sample code was a bit inconsistent between the three handlers. This PR cleans them up and makes them consistent:

• Use async handler instead of callbacks for all three
• Use AWS.DynamoDB.DocumentClient for all three
• Use promises when using DynamoDB for all three
• Supply region for DynamoDB for all three

Should be no functional difference, just cleaning it up to be consistent so it's easier to follow rather than coding style changing from file to file.

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