Fix SqlBagOStuff exptime uniqueness assumption

Fix SqlBagOStuff exptime uniqueness assumption

The WMF parser cache tables have some 30000 rows with an identical, old
expiry time. So each time deleteObjectsExpiringBefore() is run, the
first batch on each table is correct, but the keys for the second batch
are selected with exptime > X, where X is shared exptime of the many old
rows.

So, use exptime >= X instead.

Change-Id: I6853f64f88f65d4529be2a42c9ed70cfa62cf653