Migrate _termination_action into _End

For a termination action to properly mark an _End as having stopped it
must clear the _End's _cycle attribute. To be able to do that the
termination action must hold a reference to the _End instance.
Migrating the _termination_action behavior that creates termination
actions into the scope of the _End instance is the best way to afford
the needed instance reference.

src/python/grpcio/grpc/framework/core/_end.py

@@ -85,35 +85,6 @@ def _future_shutdown(lock, cycle, event):
   return in_future
 
 
-def _termination_action(lock, stats, operation_id, cycle):
-  """Constructs the termination action for a single operation.
-
-  Args:
-    lock: A lock to hold during the termination action.
-    stats: A mapping from base.Outcome.Kind values to integers to increment
-      with the outcome kind given to the termination action.
-    operation_id: The operation ID for the termination action.
-    cycle: A _Cycle value to be updated during the termination action.
-
-  Returns:
-    A callable that takes an operation outcome kind as its sole parameter and
-      that should be used as the termination action for the operation
-      associated with the given operation ID.
-  """
-  def termination_action(outcome_kind):
-    with lock:
-      stats[outcome_kind] += 1
-      cycle.operations.pop(operation_id, None)
-      if not cycle.operations:
-        for action in cycle.idle_actions:
-          cycle.pool.submit(action)
-        cycle.idle_actions = []
-        if cycle.grace:
-          _cancel_futures(cycle.futures)
-          cycle.pool.shutdown(wait=False)
-  return termination_action
-
-
 class _End(End):
   """An End implementation."""
 
@@ -133,6 +104,31 @@ class _End(End):
 
     self._cycle = None
 
+  def _termination_action(self, operation_id):
+    """Constructs the termination action for a single operation.
+
+    Args:
+      operation_id: The operation ID for the termination action.
+
+    Returns:
+      A callable that takes an operation outcome kind as its sole parameter and
+        that should be used as the termination action for the operation
+        associated with the given operation ID.
+    """
+    def termination_action(outcome_kind):
+      with self._lock:
+        self._stats[outcome_kind] += 1
+        self._cycle.operations.pop(operation_id, None)
+        if not self._cycle.operations:
+          for action in self._cycle.idle_actions:
+            self._cycle.pool.submit(action)
+          self._cycle.idle_actions = []
+          if self._cycle.grace:
+            _cancel_futures(self._cycle.futures)
+            self._cycle.pool.shutdown(wait=False)
+            self._cycle = None
+    return termination_action
+
   def start(self):
     """See base.End.start for specification."""
     with self._lock:
@@ -174,8 +170,7 @@ class _End(End):
     with self._lock:
       if self._cycle is None or self._cycle.grace:
         raise ValueError('Can\'t operate on stopped or stopping End!')
-      termination_action = _termination_action(
+      termination_action = self._termination_action(operation_id)
-          self._lock, self._stats, operation_id, self._cycle)
       operation = _operation.invocation_operate(
           operation_id, group, method, subscription, timeout, protocol_options,
           initial_metadata, payload, completion, self._mate.accept_ticket,
@@ -208,8 +203,7 @@ class _End(End):
         if operation is not None:
           operation.handle_ticket(ticket)
         elif self._servicer_package is not None and not self._cycle.grace:
-          termination_action = _termination_action(
+          termination_action = self._termination_action(ticket.operation_id)
-              self._lock, self._stats, ticket.operation_id, self._cycle)
           operation = _operation.service_operate(
               self._servicer_package, ticket, self._mate.accept_ticket,
               termination_action, self._cycle.pool)