rework cache invalidation using generation tags

[iay]

At present, the MetadataService layer maintains a cache of Results which it invalidates whenever it (periodically) asks the ItemCollectionLibrary to refresh the collection. This is suboptimal in several ways, and will tend to cause cached results to be discarded more frequently than they need to be if the ItemCollectionLibrary gets more intelligent about refresh, for example by pre-checking the source document and retaining previous results if the source document has not changed (aside: this probably requires reworking the single source pipeline into a document fetch and processing pipeline). It also doesn't work at all if several MetadataServices are clients of the same ItemCollectionLibrary.

A better solution would be to attach a generation tag (e.g., a UUID) to each IdentifiedItemCollection and have the MetadataService remember that in its cache along with the corresponding Result. The MetadataService would then revalidate cache contents by making an IdentifiedItemCollection query and comparing generation tags. If the tag matched, the cached Result would still be valid; if not, the cached Result would be discarded.

This would also move the control of refresh firmly back into ItemCollectionLibrary, which I think is preferable to the current hack.

[iay]

Done in 4f0f417 to a7cfb83.