This commit fixes two bugs. Lock in message handler of service_t is more localized now. Before this lock didn't allow to make call to service from handler of previous call to the same service object. Now lock is used only when it really needed. Second bug is that upstream (response on invoke to application) was not thread-safe. Moreover worker's communication with cocaine engine was not thread safe. So writing to one upstream or even writing to different upstreams from mulltiple threads could lead to crash. Now all communication is under lock.