Add a new table for "bucketed" object counts

This table allows counting objects by bucket, keeping the transactions for
counting objects short (a few dozen seconds at most).

Also add a "single_update" boolean field to the main object_counts table to be
able to discriminate tables that are counted via buckets and tables counted on
one go.

The main table is updated every 256 counted buckets to avoid too much churn on
the table.

Close T962.