Drop the top-level "turb" data structure

Drop the top-level "turb" data structure

In the original turbot code, this was a simple structure to hold on to
all the things necessary for turbot. Specifically, it had a boto
connection to the database, a handle for the Google sheets API, and a
handle for the Slack client API.

In this code, the only one of those things we talk to is the DynamoDB
table so that's all we need here.