Add ARCHIVAL storage type
Allow storing StorageType hint with UnderReplicatedBlocks (inefficient, first cut)
Expose BlockWithStorageHints to users of UnderReplicatedBlocks (they don't use it yet)
Pass storage hint to block placement policy. It is not respected by the block placement policy yet, nor by the DataNode
Decommission should pass correct StorageType hint to UnderReplicatedBlocks
PendingReplicationBlocks tracks StorageType hint and propagates it to UnderReplicatedBlocks