internal/postgres: dedup imported-by counts in the DB
internal/postgres: dedup imported-by counts in the DB

Deduping in RAM (the "seen" map) was consuming so much RAM
that the GAE process was OOMing.

Have the DB dedup instead.

Change-Id: Iaf5abb34cb55406f8c7c6d48bf76878d68c6deb8
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/594878
CI-Result: Cloud Build <ea5729d4cf3392b9d382b6ca272c65aa27554f44@system.gserviceaccount.com>
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite