[FTS] - partition assignment "nudging" is wrong

Thanks Sreekanth Sivasankaran – Sreekanth helped to raise attention that the cbgt "node list" nudging algorithm is wrong...

https://github.com/couchbase/cbgt/blob/master/manager_planner.go#L671
manager_planner.go:671

The idea is that it takes the indexName,
then computes a hash num,
then uses that hash num to naively rotate or shift the array of nodes around
so that each index might have a different looking but repeatable-ish array of node names as it goes into the rest of the (blance) partition planning algorithm.

The bug is here...

https://github.com/couchbase/cbgt/blob/master/manager_planner.go#L683

    if next >= len(nodeUUIDsAll) {

			next = 0

		}

It should probably more use a modulus, perhaps like...

   if next >= len(nodeUUIDsAll) {

      next = next mod len(nodeUUIDsAll)

   }

(Related, when the involved numbers are low, like there's only a single partition per index, and only two nodes, then this nudging or list rotation approach might not be enough – but we can consider that separately perhaps in some future improvement ticket.)