Add support for SynchronizationContext

[hoffmann-stefan]

Add support for SynchronizationContext to make async code easier.

• For the Prototype see this Gist: https://gist.github.com/hoffmann-stefan/33a075a5ea36d2304967248ada75e895
• Still needs work for including in rcldotnet:
  • [ ] Definie the API.
  • [ ] Integrate to executers and callback groups.
  • [ ] Provide an expample on how to use this.
  • [ ] Document how async code with a SynconizationContext can avoid multiple threads and locks for most cases.