Code review comment for lp:~albaguirre/mir/fix-1427976

So we have three options:

1. Use a BinarySemaphore as proposed here
2. Make SessionMediator::exchange_buffer (and others which advance the buffer) wait for the completion and execute done->Run synchronously, in which case we can use a std::mutex
3. Remove the session_mutex all together by removing the --ipc-thread-pool option and doing 2 above (so only a single IPC thread is active at any time)