client: fix race condition when a storage connection is closed just after identification

The consequence was that the client never reconnected to that storage node.
On commits, writes to that node always failed, causing the master to
disconnect it.

neo/client/pool.py

@@ -98,7 +98,13 @@ class ConnectionPool(object):
                         conn = self._initNodeConnection(node)
                         if conn is not None:
                             self.connection_dict[uuid] = conn
-                            return conn
+                            if not conn.isClosed():
+                                return conn
+                            # conn was closed just after the reception of
+                            # AnswerRequestIdentification (e.g. RST upon ACK),
+                            # and removeConnection may have been called (by the
+                            # poll thread) before we add it to connection_dict.
+                            self.connection_dict.pop(uuid, None)
 
     def removeConnection(self, node):
         self.connection_dict.pop(node.getUUID(), None)