Add a new method copyBlockCrossNamespace to DataNode

Add a new method copyBlockCrossNamespace in DataTransferProtocol at the DataNode Side.

This method will copy a source block from one namespace to a target block from a different namespace. If the target DN is the same with the current DN, this method will copy the block via HardLink. If the target DN is different with the current DN, this method will copy the block via TransferBlock.

This method will contains some parameters:

• ExtendedBlock sourceBlock
• Token<BlockTokenIdentifier> sourceBlockToken
• ExtendedBlock targetBlock
• Token<BlockTokenIdentifier> targetBlockToken
• DatanodeInfo targetDN