Default db_replicator per_diff is the degenerate configuration

[clayg]

The default per_diff for db replicators has been 1000 for ever basically:

https://github.com/openstack/swift/blob/master/swift/common/db_replicator.py#L197

At some point a long time ago merge_items was optimized to bulk inserts, but we're limited to 999 at a time:

https://github.com/openstack/swift/blob/master/swift/container/backend.py#L1287

Now, it's probably worth noting that often the slurp of the .pending will add a few hundred items to the list. So 1500ish might be a better default than 999.