Dynamic metadata error caching

[Kristof Bajnok]

Hi,

this is regarding https://issues.shibboleth.net/jira/browse/SSPCPP-733.

I see that during dynamic metadata fetch, the cache is populated even
though the request may fail. This makes the SP not to retry the metadata
query for minCacheDuration seconds (defaults to ten minutes).

I assume that this is implemented in order to prevent resource hogging.
I can't think of any normal configuration where it could happen, so the
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.

What do you think?

Kristof