Avoid unnecessary table scan when finding reminder entries to delete

[jdom]

There's a possibility to filter by partition key instead with the same results. This was timing out when the reminders table becomes too large.

[gabikliot]

Looks good, good catch, but we don't realy use that method. Its only uses to DeleteTableEntries, which is only used in Test. But I know understand why Test cleanup would sometimes fail for reminder tests. I suspected it has something to do with the way we inefficiently delete this table. Now its clear.