Dynamic metadata error caching

[Cantor, Scott]

> I assume that this is implemented in order to prevent resource hogging.

It's to prevent constant attempts to query what could be a hung metadata source that might be causing requests to block. The system has no way to know that the error is transitory so it resets itself to try again after a minimal duration.
 
> I can't think of any normal configuration where it could happen

Failure is just an expected possibility. It could maintain a counter I suppose, but I don't think endless retry is really the right approach.

> simplest would be to just remove lines 346-362 from
> DynamicMetadataProvider.cpp, but if you still want to deal with this, I
> think a new variable like retryDelay should be used instead, defaulting
> to something like 1 second. I don't think that lowering minCacheDuration
> would be the proper way to avoid sticking metadata errors.

It is the proper way, that's what it's for. I don't think it really has much of an impact on anything else.

-- Scott