Support Transaction

Since DynamoDB uses HTTP connection model, it's important to keep requests count as few as possible. For example, if we try to maintain "likes" count, we would do:

1. Add like if the user didn't liked yet.
2. If added, increment the counter.

We can execute them in a single request with [Transaction](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/transaction-apis.html).
