Store the cluster in the key for performance and easier customization on mappers

Right now the chukwa framework is storing the destination cluster as a tag in the Chunk. Then the tags are copied to the ChukwaRecord, and before storing it, it's parsed with a regular expression from each record.

• It's slow to apply a preg to each record
• It's harder to modify the destination cluster from the mapper, you have to tweak the tags field.
• Takes unneeded space on records storing the cluster on each of them.

The proposed path:

• Extracts the cluster from chunk tags just once per chunk, much faster.
• Stores the cluster in the key, so it's easy to recover.
• It's easy to tweak from the mapper. Just alter it with key.setClusterName(String clusterName)
• Strips the cluster from the tags field of the resulting chukwa records. If the tags field is empty, completely skips setting the tags field in the record.