Fix listShards to only specify nextToken

[Anthuang]

This fixes two bugs:

1. If listShards returns multiple pages, then we run into an infinite loop with while (!nextToken.isEmpty), because nextToken will never be set back to null because:

if (returnedToken != null) {
  nextToken = returnedToken

2. listShards cannot allow both StreamName and NextToken to be specified together.

[chadlagore]

@itsvikramagr this is tested against streams with >1k shards which forces paging on the Kinesis API

[itsmesrds]

Does this read the equal number of messages too ? because I see there is a data skew.