Problem: Missing swap functions

Solution: Implement for socket_t, context_t, message_t and poller_t
Additionally remove dependency on <functional> by refactoring poller_t
and remove unused <unordered_map> include.

tests/monitor.cpp

@@ -4,6 +4,7 @@
 #include <thread>
 #include <mutex>
 #include <condition_variable>
+#include <functional>
 
 class mock_monitor_t : public zmq::monitor_t
 {