Presently the layer and namespace tables use type `varchar(128)` for
their respective name columns. For layer, this width works fine enough
using the sha256 digests provided by docker. However, if one wishes to
encode the image name into the layer name (eg, to avoid collisions like
in [0]), the limit of 128 bytes starts to feel a bit cramped. Bump to
256 bytes, since that "ought to be enough for anybody." (TM)
[0]: https://github.com/coreos/clair/issues/319
