Add script to verify contents of the profile store

This runs through each profile ID in the `Profiles` table and checks whether
it's in the KV store. Yes, it loads all the profile IDs in one DB call - since
it's the primary key, that comes back real quick, and it's only a few MB. Didn't
feel worth spending the time on paging.
Teach profile verifier to fix the mismatch

With the `--fix` option, the script will save to the KV store any profiles not
found in it yet. It does this simply by calling `allSet` with no updates (it's
really a merge, not a set). The get performed will fall back and load from the
`Profiles` table, and then the save happens on the KV store.
Let us exit when an error is detected

When we're feeling cautious, this flag will stop the script as soon as an error
comes back from the DB or KV store.
Select all profiles from the DB

Just had this in for testing, but we want all of 'em.
Let us define how many workers run concurrently

In case we're hammering any storage, this will let us scale back.
Merge pull request #810 from Singly/verify-profile-store

Check and fix missing profiles in the KV store