Added multi-threading across tables

[opteemister]

Hello, I decided to share changes in your code in case you want to use anything from that. We wanted to configure capacity scaling for all our DynamoDB table (about 81) and we need to run check pretty frequent in case we reached throttling or unexpected high load. But one round takes about 15 minutes to check all tables and then check again. It was too much and we decided to check tables in separate threads. These changes where made to fit our needs, but you can find it useful and maybe could make it configurable.