[qa] Handle disconnect_node race

Several tests call disconnect_nodes() on each node-pair in rapid
succession, resulting in a race condition if a node disconnects a peer
in-between the calculation of the nodeid's to disconnect and the
invocation of the disconnectnode rpc call.  Handle this.

test/functional/test_framework/util.py

@@ -351,7 +351,14 @@ def set_node_times(nodes, t):
 
 def disconnect_nodes(from_connection, node_num):
     for peer_id in [peer['id'] for peer in from_connection.getpeerinfo() if "testnode%d" % node_num in peer['subver']]:
+        try:
             from_connection.disconnectnode(nodeid=peer_id)
+        except JSONRPCException as e:
+            # If this node is disconnected between calculating the peer id
+            # and issuing the disconnect, don't worry about it.
+            # This avoids a race condition if we're mass-disconnecting peers.
+            if e.error['code'] != -29: # RPC_CLIENT_NODE_NOT_CONNECTED
+                raise
 
     # wait to disconnect
     wait_until(lambda: [peer['id'] for peer in from_connection.getpeerinfo() if "testnode%d" % node_num in peer['subver']] == [], timeout=5)