[experimental] async exec requests with xeus

[DerThorsten]

In a nutshell:

• new async handlers in kernel-core (atm only execute_request)

• new xainterpreter class:

  • this is just the old interpreter class, but :
    • async_execute_request instead of execute_request method
    • async_execute_request gets a response_sender as additional argument
    • returns void instead of a reply
    • callers of async_execute_request are responsible for calling response_sender with the reply instead of returning it

• backward compatible / should not break existing code bases:
  - new xinterpreter class deriving from the new xainterpreter having the old API

• atm the publisher story is very hacky:

  • all the methods like "display_data" / "publish_execution_error" do not work by default because they use the wrong parent header in case of concurrent requests (jeah xeus stores parent headers in a single variable which is overwriten on concurrent requests)
  • => as a quick hack I added overloads for these methods that a parent-header can be passed but this is a bit ugly

[DerThorsten]

#384 is using most of the ideas of this pr