Create Service::dispatch

[ruifig]

This would be the equivalent of Asio's service::dispatch.
Unlike Service::post, dispatch will execute the handler right away if all the czspas guarantees can be met.

This requires adding a Callstack class similar to what I have in the article http://www.crazygaze.com/blog/2016/03/11/callstack-markers-boostasiodetailcall_stack/