experimental: reimplement operator graph with coroutines

[bigerl]

evaluate feasibility of re-implementing the operator graph using co-routines.
https://github.com/lewissbaker/cppcoro might be interesting.
careful: we need to switch to libc++ (instead of libstdc++) to be able to use it
Boost.Coroutine2 should NOT be used

if feasible implement and evaluate the performance with tentris paper benchmarks