repo-updater: Find root cause of memory spikes

Created by: tsenart

We've had a few incidents the last two days with repo-updater running out of memory. Profiling shows most of the memory usage comes from SyncExternalService -> Store.ListRepos, but I still don't understand exactly how it reaches so many GBs.

https://docs.google.com/document/d/1dtrOHs5STJYKvyjigL1kMm6u-W0mlyRSyVxPfKIOfEw/edit?ts=5fd3c02f#heading=h.uowbrn97vdhn

https://console.cloud.google.com/profiler;timespan=7d;end=2020-12-11T21:32:00.000Z/repo-updater;type=HEAP;weight=99/inuse_space?project=sourcegraph-dev

https://sourcegraph.com/-/debug/grafana/d/repo-updater/repo-updater?viewPanel=37&orgId=1&from=now-7d&to=now