ddfs_tag: avoid an extra temporary variable.
ddfs_tag: encode the tag contents at the slave node.

Technically, doing this on the master is better because it avoid
encoding/decoding just for transferring the contents. However, we can do better
on the slave nodes when we have the whole structure.
ddfs_tag: avoid sending the tag id separate from the tag content.

The tag content includes the tag id and the id can be looked up whenever needed.
mnesia for tags part 1.