dynamic metadata caching.

[Rod Widdowson]

Me on Dynamic Metadata providers again.  Sorry.

The current SP has a positive and negative caching of lookups driven by the min/max CacheDuration and refreshDelayFactor.   I'm
absolutely looking to continue this for the Http providers (MDQ and Dynamic).

My question is whether (or rather how) we should do this for the LocalMetadata provider?  AFAICS the IdP does have this control, but
it doesn't cache a cache-miss, so the behavior on "pop a file in to a directory and try again" is different.

We need some sort of control to age out old metadata (from memory), but I'm slightly concerned about the impact of the negative
lookup cache on the semantics of the local lookup.

Best thing I can come up with is to control negative caching either with implicitly (on for HTTP, off for local) or explicitly (with
defaults as for the explicit case).

Thoughts?

Rod