Split SourceAPI into two subobjects.
Parallel structure for endpoints, services will allow
changing to use generic code in pkg/client/cache/reflector.go.
Rename some funcs to be more like pkg/client/cache.
Allows us to define different watch versioning regimes in the future
as well as to encode information with the resource version.
This changes /watch/resources?resourceVersion=3 to start the watch at
4 instead of 3, which means clients can read a resource version and
then send it back to the server. Clients should no longer do math on
resource versions.