This is a trivial change, internally Butler uses `_registry` but it is also
exposed as `registry` property. In the next commits `registry` property
will return a special shim object, this will allow us to transparently
update registry implementation without breaking client-visible interfaces.

`Registry` ABC now contains methods used by regular registry clients, this
interface is exposed via `Butler.registry` property. `ButlerRegistry` ABC
extends `Registry` with methods that are used by Butler, this includes
factory methods and a couple of internal methods. SQL and remote
implementations now inherit `ButlerRegistry`, they are not changed otherwise.

Also dropped `Registry.datasetIdFactory` attribute, we now have more transparent
method for generating dataset IDs and this attribute is not used.

This new class implements `Registry` interface and forwards all methods
to other objects. Everything is forwarded to `Butler._registry` now, but
this will change as we start refactoring ButlerRegistry implementation.

Trying to further separate concerns into independent classes. Re-introduced
`Registry.createFromConfig` to avoid updating tests in many other packages.