Refactor ReleaseDB schema (and name)

[mjuric]

The SQL schema used by the cache of built packages is a horrible mess. It needs to be refactored and normalized. I think the right schema is something along the lines of:

Products:
    prod_id, name, version
Packages:
    pkg_id,   prod_id, channel_id,   buildnum, filename, recipe_hash
Channels:
   channel_id, urlbase

with a many-to-many relationship from Products->Packages and a one-to-many from Channels->packages

While at it, the ReleaseDB class should also be renamed to something like BuiltProductsCache.

[mjuric]

ReleaseDB has been renamed to RecipeDB