Saving execution data is one of the slowest DB operations in the
application, and is likely behind some of the sqlite transaction
concurrency issues we've been seeing.
This not only remove the 2 separate transactions for saving
`ExecutionEntity` and `ExecutionData`, but also remove fields from
`ExecutionData.workflowData` that don't need to be saved (like `tags`,
`shared`, `statistics`, `triggerCount`, etc).
