Removed unnecessary import
Instrumented the WritableRpcEngine.  I'm not sure where this is used (as
opposed to the Protobuf engine) but Todd's htrace instrumentation chose
to instrument this class too, so I might as well include it.
Add XTrace metadata to the data transfer header protos
Start tracing commands when initiated from the filesystem.
Changed the DataNode receiver to start traces or join traces.
Added XTrace metadata to the block and packet transfer headers
Instrumented the sender/receiver for downloading files from HDFS