There's synchronization around `RouterStubManager.stubs` that's not needed: `stubs` is never changed after initialization, so synchronization can be removed. Possibly use a CopyOnWrite list or a concurrent list

`RouterStub.writeRequest()` is also synchronized, but it calls either `TcpConnection.send()` or `NioConnection.send()`, which are both protected by a lock, so we can remove synchronization on `writeRequest()`.