Deletion of item that hasn't been persisted before should be persisted in couchstore to support XDCR

Consider the following sequence of operations:

SET (k1, v1)
DEL(k1)

Assume that k1 doesn't exist before. The first SET is still in the checkpoint queue, and is de-duped by the DEL. The flusher will skip the DEL operation as "k1" was never persisted before. This could be an issue for XDCR. In this case, we still need to persist the deletion of "k1" into couchstore.