Delete purgemarker when purging data in background

[manish-sethi]

In purge private data, in the initial design we considered keeping one purge marker that would be used for both a) deleting the data in the background and b) reconciliation. However, this adds inefficiency in the background goroutine as we need to keep the purge marker for reconciliation forever. So, the proposal is to keep two copies of the purge markers for both the above purposes.