insights: lock aggregation updates

Created by: leonore

closes #41423 (closed)

we think that we didn't notice this unsafe behaviour sooner because we expected type:commit queries to time out, and commit search triggers a search of each repo in a separate go routine which would have triggered this concurrent access

Test plan

Ran a type:commit query locally and observed it worked rather than panicked the whole instance

Also tested the sync lock on a lower level in another branch with go test -race

we went with the safety of a lock at the higher level function (to also align with search's implementation)