MB-100: moved samples_loader_tasks earlier in ns_server_sup list

Because ns_heart is asking it for list of tasks it makes sense to
start it before ns_heart.

Change-Id: I2051bacc23b07e37d6185b00c033a4c3ceca5fb4
Reviewed-on: http://review.couchbase.org/25459
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>