ItemCollectionLibrary should be able to suppress refresh

[iay]

At present, a #refresh() on ItemCollectionLibrary unconditionally runs the source pipeline. This means that the library will be updated whether anything has changed or not, which in turn means that it may change more often than necessary.

There are a couple of ways to approach this. One is to discard the results of the source pipeline if it exactly matches the results of the previous run.

Another is to suppress running the source pipeline if we know (through evaluation of a configurable predicate) that the result will be the same. In the usual case, the predicate could be made up from checks on the last modified date or ETag of each of the documents which will be accessed by the source pipeline.