Nested aggregation collects docs out-of-order

[jpountz]

This issue is easier to explain with an example: let's imagine that doc ID 0 and 1 are nested documents of doc ID 2. If the parent collector calls the nested aggregator with doc ID 2 and buckets 0 and 1 then the nested aggregator will call its sub aggregators with the following docID, bucketOrd tuples:

• docID= 0, bucket=0
• docID=1, bucket=0
• docID=0, bucket=1
• docID=1, bucket=1

I think we should either buffer all collected buckets per doc ID or change the nested aggregator to have a different sub aggregator instance per bucket?