RFC: centralise registries

[Ian Clatworthy]

Robert Collins wrote:

> We could make all the modules that have registries cheap to load, or we
> could move all the registries to bzrlib.registry, which is easier to do
> as we don't need to deal with subclasses or other such things happening
> there.

The Registry class is a great support class and arguably useful one day
outside of bzrlib so I'd prefer registry.py didn't know more about the
rest of application than it needs to.

Moving all registries to bzrlib.registry is the wrong thing to do IMHO.

Ian C.