MB-7902: Log additional info when nodedown is seen

Request net_kernel to supply a reason when sending nodedown
messages and log them, to aid debugging unexplained disconnects
during rebalance.

Change-Id: Ic4ef710b393e63c03385776e834d424c2119c396
Reviewed-on: http://review.couchbase.org/25297
Tested-by: Sriram Melkote <siri.couchbase@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>