MB-10286: Revert "MB-100 slight ns_rebalancer re-factoring"

This reverts commit 6d02762e6d3f72dced7e2501ab984356085e3d4b.

Somehow (I haven't bothered to investigate) original commit broke
rebalance out.

Change-Id: Idb17a7e58a4fc675a75b17d25eca826449587749
Reviewed-on: http://review.couchbase.org/33887
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>