Implement two-phase issue import/update

[mdittmer]

Let NewIssues be the list of issues created by running the issue generator.

Let ExistingIssues be the prod database with persistent state.

To update the prod database:

• Push any issue in NewIssues and not in ExistingIssues
• Mark as CLOSED_UNVERIFIED any issues in ExistingIssues and not in NewIssues

[mdittmer]

@foolip and @jpmedley this is predicated on a persistent, stateful approach to issues. Do you think we've reached a consensus that that's what we want (and we should therefore doe something like this to "auto-curate" fixed issues)?

[jpmedley]

I agree.