Raise StopIteration when iterating over an already-completed Stream (typedb#185)

## What is the goal of this PR?

Previously, iterating over an already-completed Stream would stall. Now, we instead raise a `StopIteration` error.

## What are the changes implemented in this PR?

Raise StopIteration when iterating over an already-completed Stream

Author: Alex Walker

grakn/rpc/stream.py

@@ -34,11 +34,14 @@ def __init__(self, transaction, request_id: str, transform_response: Callable[[t
         self._request_id = request_id
         self._transform_response = transform_response
         self._current_iterator = None
+        self._done = False
 
     def __iter__(self):
         return self
 
     def __next__(self):
+        if self._done:
+            raise StopIteration()
         if self._current_iterator is not None:
             try:
                 return next(self._current_iterator)
@@ -54,6 +57,7 @@ def __next__(self):
             self._transaction._request_iterator.put(continue_req)
             return next(self)
         elif res_case == Stream._DONE:
+            self._done = True
             raise StopIteration()
         elif res_case is None:
             raise GraknClientException("The required field 'res' of type 'transaction_proto.Transaction.Res' was not set.")