Better sync-write status when DCP is down

If DCP is disconnected for a prolonged period (greater than a sync-write timeout) a sync-write will return status "sync-write-ambiguous". However in the case where the active vbucket cannot replicate anything because no ActiveStream or DCPProducer object exists, a better status code could be determined. The active durability code should have everything at hand to be able to determine that no DCP_PREPARE message has left the node and that any timeout of the sync-write is 100% not at all ambiguous - what status code is appropriate though is unclear, the write maybe durable on the local node (persisted) so it's not fair to say the write was 100% failed.