Store data and checksums together in block file

The current implementation of HDFS stores the data in one block file and the metadata(checksum) in another block file. This means that every read from HDFS actually consumes two disk iops, one to the datafile and one to the checksum file. This is a major problem for scaling HBase, because HBase is usually bottlenecked on the number of random disk iops that the storage-hardware offers.