Truncated Response Handling in Route53 ListResourceRecordSets

According to the [Route53 Documentation](http://docs.aws.amazon.com/Route53/latest/APIReference/API_ListResourceRecordSets.html), when a response is truncated, we could get a `NextRecordIdentifier` element.

We encountered an instance where one weighted resource recordset was returned as the last element of the response, and boto did not take `NextRecordIdentifier` into account so the record was returned again in the next iteration of the GET.

Example snippet:

```
...
    <ResourceRecordSet><Name>example.foo.com.</Name><Type>A</Type><SetIdentifier>primary</SetIdentifier><Weight>100</Weight><TTL>1800</TTL><ResourceRecords><ResourceRecord><Value>127.0.0.1</Value></ResourceRecord></ResourceRecords></ResourceRecordSet>
  </ResourceRecordSets>
  <IsTruncated>true</IsTruncated>
  <NextRecordName>example.foo.com.</NextRecordName>
  <NextRecordType>A</NextRecordType>
  <NextRecordIdentifier>secondary</NextRecordIdentifier>
  <MaxItems>100</MaxItems>
</ListResourceRecordSetsResponse>

DEBUG:boto:Method: GET
DEBUG:boto:Path: /2013-04-01/hostedzone/XXXXX/rrset?type=A&name=example.foo.com.
DEBUG:boto:Data:
DEBUG:boto:Headers: {}
DEBUG:boto:Host: route53.amazonaws.com
DEBUG:boto:Port: 443
DEBUG:boto:Params: {}
DEBUG:boto:Token: None
DEBUG:boto:Saw HTTP status: 200
DEBUG:boto:<?xml version="1.0"?>
<ListResourceRecordSetsResponse xmlns="https://route53.amazonaws.com/doc/2013-04-01/">
  <ResourceRecordSets>
    <ResourceRecordSet><Name>example.foo.com.</Name><Type>A</Type><SetIdentifier>primary</SetIdentifier><Weight>100</Weight><TTL>1800</TTL><ResourceRecords><ResourceRecord><Value>127.0.0.1</Value></ResourceRecord></ResourceRecords></ResourceRecordSet>
```

Notice the that the "example.foo.com." record was a duplicate.


This patch fixes the issue by remembering the `NextRecordIdentifier` value (in addition to `NextRecordName` and `NextRecordType`) in the first request and sending it in the second GET request.