DFSClient read performance can be improved by stagerring connection setup to datanode(s)

The DFS client opens a socket connection to a DN for the n-th block, fetches n-th block from that datanode and then opens socket connections to the datanode that contains the n+1th block. Sequential-reads might show performance improvements if the setting up of socket connections to the datanode containing the n+1th block can happen in parallel while the data for the nth block is being fetched. The amount of improvement, if any, has to be measured.