Add `ConfigProto.isolate_session_state` option for the distributed runtime.

Setting this option to true when creating a session ensures that no
stateful resources (variables, queues, iterators, etc.) will be
visible to any other session running on the same server, and those
resources will be deleted when the session is closed.

The default behavior, namely that all `tf.Variable` objects are shared by
default and most other resources are shared when their `shared_name` attr is
non-empty, is preserved.

This change augments the semantics of the WorkerService.CreateWorkerSession
RPC. Now, if the server_def in the request is empty, it implies that
the worker should use its default ClusterSpec. Note that clusters created
using ClusterSpec propagation always have isolated session state, and are
unaffected by this change.

PiperOrigin-RevId: 177173545