Set the shutdown event via call_soon

Fixes #480

Require aiorpcx 0.5.6, handle ConnectionError appropriately;
this ensures the same bug would give a better error message

server/controller.py

@@ -591,7 +591,7 @@ class Controller(ServerBase):
 
     def rpc_stop(self):
         '''Shut down the server cleanly.'''
-        self.shutdown_event.set()
+        self.loop.call_soon(self.shutdown_event.set)
         return 'stopping'
 
     def rpc_getinfo(self):

server/peers.py

@@ -16,7 +16,7 @@ import time
 from collections import defaultdict, Counter
 from functools import partial
 
-from aiorpcx import ClientSession, RPCError, SOCKSProxy
+from aiorpcx import ClientSession, RPCError, SOCKSProxy, ConnectionError
 
 from lib.peer import Peer
 from lib.util import ConnectionLogger
@@ -73,7 +73,7 @@ class PeerSession(ClientSession):
     def is_good(self, request, instance):
         try:
             result = request.result()
-        except asyncio.CancelledError:
+        except (asyncio.CancelledError, ConnectionError):
             return False
         except asyncio.TimeoutError as e:
             self.fail(request, str(e))