Call stats recorder asynchronously 
From what I can tell miningcore collects the stats from each pool sequentially.

Do you think it would make sense to call `UpdatePoolHashratesAsync` for each individual pool and await them using `Task.WhenAll()`? 

https://github.com/oliverw/miningcore/blob/2311784f0eb5e5f6180a5ac12eec2c9a270f9ca5/src/Miningcore/Mining/StatsRecorder.cs#L100