WIP: Async query of mistral results.

I just opened a WIP PR so you can provide me early feedback. Please focus on the idea and not coding problems. I'll add tests etc and pass it through linter :). 

## TODO
[X] - Create a boiler plate Querier class which makes query on eventlets.
[X] - Provide an interface for implementing query
[X] - Save the query results in ActionExecutionDB
[X] - Data models for persisting action execution states. 
[ ] - Provide a mechanism to bootstrap the querier with query contexts after restart
[ ] - Provide a mechanism to add a query context in a live system [wiring]
[ ] - Add tests
[ ] - Test one box deployment
I can figure that mistral exec_id is coming from the context while our execution_id is coming from parameters, but this name match is confusing.