support incremental verification when retrieving from export/import remotes

support incremental verification when retrieving from export/import remotes

None of the special remotes do it yet, but this lays the groundwork.

Added MustFinishIncompleteVerify so that, when an incremental verify is
started but not complete, it can be forced to finish it. Otherwise, it
would have skipped doing it when verification is disabled, but
verification must always be done when retrievin from export remotes
since files can be modified during retrieval.

Note that retrieveExportWithContentIdentifier doesn't support incremental
verification yet. And I'm not sure if it can -- it doesn't know the Key
before it downloads the content. It seems a new API call would need to
be split out of that, which is provided with the key.

Sponsored-by: Dartmouth College's Datalad project