Update SetResponse error and timestamp.
Update SetResponse error and timestamp.

  * (M) rpc/gnmi/gnmi.proto
    - SetResponse utilised an Error message, which duplicates the
      contents of google.rpc.Status when returned. This change
      deprecates the top-level Error, preferring to use rpc.Status.
      If additional details are to be included by the target when
      returning details to the server, then the google.protobuf.Any
      field of rpc.Status can be used to store a gnmi.Error message.
      This change reduces complexity at the client which otherwise
      must handle the case that the error field indicates an
      incomplete Set RPC, but the grpc.Code in the return indicates
      success.
    - Deprecate the timestamp field of the UpdateResult message.
      gNMI defines all operations within a Set RPC to be a single
      transaction, therefore it is ambiguous what this timestamp
      implies - since validation can only be performed at the end
      of the transaction.
    - Add a new timestamp field to the SetResponse message to
      timestamp the overall transaction.

openconfig/reference,openconfig/reference