[WIP] Migrate to Spring Webflux

[geofjamg]

• Do you want to request a feature or report a bug?
  Feature

• What is the current behavior?
  Spring boot web service is implemented with classical Spring web framework..

• What is the expected behavior?
  Spring webflux could be used instead and would allow on client side to easily parallelize queries.

• What is the motivation / use case for changing the behavior?

• Please tell us about your environment:

  • PowSyBl Version: ...
  • OS Version: ...

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)

(if a question doesn't apply, you can delete it)