Workaround for DCP backfill issue for CE 3.0.1 and 3.0.2

We fixed a DCP backfill issue in 3.0.3:
https://issues.couchbase.com/browse/MB-13386

For users on prior releases, we need to provide a workaround.
Summary of the problem:
The problem is when the last item in a vbucket is a deleted and no new updates to that vbucket happen for 7 days. The problem occurs when the compactor purges that last delete because a backfill never receives an item for the highest sequence number on disk.

The workaround:
You can add a dummy item with highest sequence number and never delete it.
Also you need to update the dummy item frequently to keep it as the item with highest sequence number.
It should be good enough to update this just before a compaction is run.