Make it possible for BlockPlacementPolicy to return null
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                 Key: HDFS-1351
                 URL: https://issues.apache.org/jira/browse/HDFS-1351
             Project: Hadoop HDFS
          Issue Type: Test
          Components: name-node
    Affects Versions: 0.22.0
            Reporter: Dmytro Molkov


The idea is to modify FSNamesystem.chooseExcessReplicates code, so it can 
accept a null return from chooseReplicaToDelete which will indicate that 
NameNode should not be deleting extra replicas.

One possible usecase - if there are nodes being added to the cluster that might 
have corrupt replicas on them you do not want to delete other replicas until 
the block scanner finished scanning every block on the datanode.

This will require additional work on the implementation of the 
BlockPlacementPolicy, but with this JIRA I just wanted to create a basis for 
future improvements.

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