Determine if datastore.key.Key should be immutable

[dhermes]

From TODO:

  # TODO: Determine if this really should be immutable.

See datastore.key.Key for relevant code and #146 for some context.

git blame output

dec0e7c6 (JJ Geewax 2014-01-28 15:15:40 -0500   9)   # TODO: Determine if this really should be immutable.

[tseaver]

Duplicate of #3.