Send the EC schema to DataNode via EC encoding/recovering command

Discussed with Uma Maheswara Rao G and [~vinayrpet], we should also send the EC schema to DataNode as well contained in the EC encoding/recovering command. The target DataNode will use it to guide the executing of the task.

Another way would be, DataNode would just request schema actively thru a separate RPC call, and as an optimization consideration, DataNode may cache schemas to avoid repeatedly asking for the same schema twice.