MB-100 some ns_single_vbucket_mover re-factoring

for better readability

Change-Id: I56b53ade7144596cbb8e8ddc665cd8e43e882d9b
Reviewed-on: http://review.couchbase.org/29959
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>