Support transactions

[darrenklein]

As of December, 2018, Dynamo supports transactions - we'd like to support these in the adapter as well.

[darrenklein]

There's currently a PR open in ex_aws_dynamo that may help with this.

[darrenklein]

See docs at https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/transaction-apis.html#transaction-apis-txwriteitems

Very important!

[darrenklein]

ex_aws_dynamo supports transactions as of version 2.3.0