Remove 0xDEAD hack.

Some tiles are not "generatable", which means that they come from
a source which this Burritile cluster isn't able to render. For
example, pre-generated aerial or terrain tiles. In this case, and
upon failure, the last modified timestamp was set to 0xDEAD.

This patch removes the need for that hack by adding an
`is_generatable` flag to the `tile_protocol` object, which
indicates the same thing without the need for egregious hackery.