FTS Rebalance may enter an infinite loop with parallel index deletions in some racy situations

With index deletions happening during a rebalance operation, there is a chance for un breakable looping while attempting to "grabCurrentSample" around https://github.com/couchbase/cbgt/blob/master/rebalance/rebalance.go#L932. 

One solution would be, enhance the sample look up error handling inside the "grabCurrentSample" function and process the return errors from "grabCurrentSample" inside looping logic.