internal/postgres: use BulkInsertReturning instead of getPathIDs
internal/postgres: use BulkInsertReturning instead of getPathIDs

Fixes b/154309056

Change-Id: I8308d813c8de6c053812425743d5c1f2b100d288
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/720885
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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