Layer deletion

[matko]

Layers can't actually be deleted right now. There are many cases when this'd be desirable.

Problems with deleting layers is that someone may be using it, and rely on its existence, which is not something store can really know. Therefore this operation would be a little bit unsafe. Still, it needs to be provided because systems building on top of store may know conditions under which layer deletion is safe.

Besides the layer deletion operation itself, code should be changed to ensure an unexpected missing layer returns an error, rather than crash the server. An unexpected missing layer may occur when

• it is the parent of an existing layer that we're loading
• it is pointed at by a label file
• it is pointed at by a rollup file

These cases should check properly for the existence of a layer and not panic.