MB-9451: serialize diag_per_node invokations

I.e. so that it's not consuming excessive amounts of CPU and/or
memory.

Change-Id: Ied973771c219656ed94a2f827a7277f2f42377b3
Reviewed-on: http://review.couchbase.org/31693
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>