Fixed race caused by writing bool &is_duplicate_normal_connection back up call stack across threads. Fixed by using std::atomic<bool> instead.

Passes make ExperRudp and TESTrudp.
2014-03-28 ned: There is a race here when checking on_connection_lost_ for
                validity as LocalFunctorReplacement<OnConnectionLost>::
                ~LocalFunctorReplacement() resets the slot without a mutex.
                I have suppressed this race as it is benign.
Added missing #include.