Spotted by Darius.

Now that both are equivalent, we can skip the sanitization part before
the yaml call.

This changes the location of source refs from `cache/cas/refs` to
`cache/sources_protos` and fixes tests that looked in the old reference
place. Source cache now uses the new protocol buffer and grpc methods
defined, for storing locally, and pushing and pulling to remotes.

Part of #1038

Instead of having methods in Context forward calls on to the Messenger,
have folks call the Messenger directly. Remove the forwarding methods in
Context.

Remove the need to pass the Context object to message handlers, by
passing what is usually requested from the context instead.

This paves the way to sharing less information with some child jobs -
they won't need the whole context object, just the messenger.

This removes the --fetch option of bst source checkout, always enabling
fetch support. This is in line with fetching subprojects as needed.