Should we limit concurrency? the amount of method calls in flight to a single 
grain at a time

for safety of the code being called there may be a reason to limit the amount 
of method calls active at any one time, this matches the Orleans concurrency 
model but is at odds with the asyncio model. it does simplify the implementation 
of the called code as no locking is required and __could__ simplify the 
scheduler (despite the complexity being written already) at the cost of 
performance (interleaving of requests and execution of other methods on blocking 
behavior)

Caveats:
Other propsals and features rely on specific aspects of the concurrency model 
and as such any implementation will need some disciossion before implementation

Ideas:
implement this at a class decorator level and make it opt in
