/** The class is responsible for choosing the desired number of targets
* for placing block replicas.
* The replica placement strategy is that if the writer is on a datanode,
* the 1st replica is placed on the local machine,
* otherwise a random datanode. The 2nd replica is placed on a datanode
* that is on a different rack. The 3rd replica is placed on a datanode
* which is on the same rack as the **first replca**.