gh-113538: Add asycio.Server.{close,abort}_clients (redo) (#116784)

These give applications the option of more forcefully terminating client
connections for asyncio servers. Useful when terminating a service and
there is limited time to wait for clients to finish up their work.

This is a do-over with a test fix for gh-114432, which was reverted.

Lib/asyncio/events.py

@@ -175,6 +175,14 @@ class AbstractServer:
         """Stop serving.  This leaves existing connections open."""
         raise NotImplementedError
 
+    def close_clients(self):
+        """Close all active connections."""
+        raise NotImplementedError
+
+    def abort_clients(self):
+        """Close all active connections immediately."""
+        raise NotImplementedError
+
     def get_loop(self):
         """Get the event loop the Server object is attached to."""
         raise NotImplementedError