MB-100: use proc_lib:spawn_link

So that crashes can be diagnosed easier.

Change-Id: I9ccf0762db41d215a0d6f2a94dd32a7fb1b6bb1c
Reviewed-on: http://review.couchbase.org/26352
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>