XDCR: Fix deletion handling of non-existent items in ep_engine

Currently, due to deduping inside ep-engine, if one or more sets are immediately followed by a delete, the sets gets overwritten by the delete. However, when the delete reaches couchstore, it will simple ignore it as there's nothing to delete. In the context of xdcr, however, it is important that we do not forget the delete.

A simple solution would be for couchstore to create a deleted document with the specified revision in Couch instead of ignoring it.