Improve scaling of downcalls using MemorySegments allocated with shared arenas

[Maurizio Cimadamore]

> sum() is really just a snapshot, it adds up the counters (Cells), so 
> it wouldn't ensure the counter was at zero. Immediately after 
> returning zero a thread could have already incremented it.
Yes. What I mean is: you can check if close() should throw because of 
pending acquires. But, as I said, we can use that in any way to "block" 
other acquires from happening in case we _do_ want to close. Which 
leaves us exposed.
>
>
>> For the purpose of implementation clarity -- would it be useful to 
>> wrap the various counters plus logic to acquire/ release (and 
>> "closing" state) into a separate abstraction, which is then used by 
>> SharedMemorySession? A sort of "atomic" LongAdder, if you will :-)
>>
>> That might make it easier to verify the correctness of the 
>> implementation, by validating each aspect (the atomic long adder, and 
>> its use from SharedMemorySession) separately.
>
> Sure, that would be a bit cleaner, thanks.

Thanks.


Maurizio