internal/postgres: update imported-by counts in smaller batches
internal/postgres: update imported-by counts in smaller batches

The worker periodically updates search_documents.imported_by_count
from the imports_unique table. It does so in batches to avoid locking
search_documents for too long.

Currently the batch size causes the update query to time out, so
reduce it.

Change-Id: If86d272f0677dbedab104e2e1e235d7f2e1e4b25
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/406296
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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