Create in-mem cache of exported emails

[aarongable]

To reduce the volume of duplicate email contact addresses that we export, the email-exporter should keep an in-memory cache of hashes of addresses it has already processed.

It's fine for this to be dropped when the exporter restarts, and for it to be duplicated across instances, since we expect to get very large benefits even with those inefficiencies. If it turns out we need even bigger wins, we can move the cache to redis later on.