Fix a race condition where a reply is received before setting the callback on an async request to handle it.

In such a case, rpyc would treat the early reply as if it was a reply for a sync request, and so the async request will be stuck till encountering a timeout
The callback needs to be registered before sending the request.
Merge pull request #136 from fviard/fix_racecondition

Fix a race condition where a reply is received before setting the callback...