Log/report node identifiers that cause import to switch to stashing and sysview import, also report on slow stashing progress

It would be useful for debugging to find out what UUIDs did not match when FV decides to use NodeStashing to overwrite a node with a new UUID.

(for instance, it would be interesting to see whether the UUID values just flip-flop between two alternatives, or whether they are really "new" each time)

Reporting on progress would be useful because stashing operations on large collections can be really slow on Oak's MongoDocumentStore.