Improve scan-proofs

[mlsteele]

1. In the case where a sighint doesn't exist on the server, that error was getting dropped on the floor. It should have been marked as a client failure. This actually probably makes up a good portion of the server=no,client=ok cases that I was seeing. Those were the reason for making proofd check PERM_FAILUREs in the first place. So, whoops. Some of those were valid, just not the ~5000 count that were due to this bug. Anyway, fixed.
2. Saving the cache is pretty slow because it's 16MB (~2s). This update makes it save only when something has changed. Sqlite would be better, but I don't it's worth it.
3. Skip deleted users.

r? @maxtaco

[maxtaco]

👍

[maxtaco]

Sorry this took so long!