Allow continuation of a reindex

[whikloj]

JIRA Ticket: https://fedora-repository.atlassian.net/jira/software/c/projects/FCREPO/issues/FCREPO-3907

What does this Pull Request do?

Adds a new flag to allow the continuation of a failed re-indexing.

How should this be tested?

In theory you need a lot of objects.

1. Start up the site either with empty indexes to with fcrepo.rebuild.on.start=true set.
2. Stop or wait for it to fail (assuming it does normally fail)
3. Set fcrepo.rebuild.continue=true, you can remove the fcrepo.rebuild.on.start flag but this flag takes precendence so you will still be okay.
4. Start Fedora.

The messages will now show a count of those skipped items as well as successful and errors. At the end you should have all your items available.

Interested parties

Tag (@ mention) interested parties or, if unsure, @fcrepo/committers

[mikejritter]

I built and tested this to confirm that it works. I ended up removing some of the rows from the index and then used the flag to trigger the rebuild.

Everything else looks good, just one question that I don't think will be an issue but figured I'd ask about.