Parallel data/socket writing for DFSOutputStream

Currently, as DFS clients output blocks they write the entire block to disk before starting to transmit to the datanode. By writing to disk the client is able to retry a block write if the datanode files in the middle of a block transfer. Writing to disk and then to the datanode adds latency. Hopefully, the common case is that block transfers to datanodes are successful. This patch writes to the datanode and the disk in parallel. If the write to the datanode fails, it falls back to current behavior.

In my tests of transmits of 237M and 946M datasets using -copyFromLocal I'm seeing a 20-25% improvement in throughput.