Support for dynamo transactions

[GrahamCampbell]

They are needed in order to properly fix #1028. There remains a race condition.

[Nyholm]

Are you referring to the Laravel cache integration or the dynamodb client?

[GrahamCampbell]

Support for transactions in the dynamodb client. They'd be nice in their own right, but would also allow us to implement releasing of locks in the async aws dynamo laravel cache driver properly. :)

[GrahamCampbell]

Transaction support is implemented.