MB-9048: use net_kernel:monitor_nodes for {d,r}doc replication

Because previously used ns_node_disco_events are aggregate events that
add multiple events and may cause events that add up to nothing to be
lost.

In order to avoid affecting semantics of this I've decided to change
{d,r}doc replication services to use nodeup/nodedown events from
net_kernel.

Note we're continuing to duplicate code between this two
services. Original reason of duplicating those facilities due to them
being too primitive and most likely needing change remains. Because
it's still not clear what we will end up doing for replicating them
I've decided to keep that small amount of duplication between this two
services.

Change-Id: Iced8e91beecf58432fddc880d02b0a2ea0f01b58
Reviewed-on: http://review.couchbase.org/28775
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>