text: Title: Top‐level mutation selection set should be executed serially
  Body: Top‐level mutation selection set should be executed serially (cf. [GraphQL spec](https://facebook.github.io/graphql/June2018/#sec-Mutation)). More information about normal & serial execution [here](https://facebook.github.io/graphql/June2018/#sec-Normal-and-Serial-Execution).
  
  Actually, all top-level selection set are parallelized. 
  
  * **Tartiflette version:** 0.3.6
  * **Python version:** 3.6.1
  * **Executed in docker:** No
  * **Is a regression from a previous versions?** No