Socket should have close() method

[oliora]

There should be a way to close a socket (i.e. cancel all async operations and cleanup all resources associated with it, especially ZMQ socket) without calling a destructor. This is extremely useful in multi-threaded applications which tend to spread shared pointers. In such scenario, user has no simple control over socket object lifetime but may need to close it at exact point.
This story is somehow related to #77.