Make the search_index command resumable

[mlissner]

We have a database with millions of items and we're using the search_index command to add the content to elastic search.

We'd like to make it so that the script can be resumed if it crashes, since that's happened to us a few times.

I don't think that's currently possible, but if it's a welcome feature (and not too much work, we haven't looked at the code too much yet), we could probably submit a PR.

Is this welcome and useful?

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Some background: We're moving from Solr, for which we have a very custom management command. It can:

• Update or delete
• Only do particular PKs
• Resume operation after a given PK
• And a few more things

We may just have to build ourselves a new custom command (perhaps an extension of this one), but it'd be great to add our features upstream if it makes sense.

Thank you!