Fix a data race and segfault.

* Configuration machine uses proper reactor now. Before I accidentally passed wrong reactor to it, so my single-threaded code runned in two threads actually.
* Don't remove service client from the callback. As I know, most Cocaine IO objects shouldn't be deleted from the callbacks called by them. The service client is not an exeption from this rule.
* Unbind a readable_stream when an socket error occurres. It's because we don't want receive any events and anything with an invalid stream.