refactor to more idiomatic RAII

* FairMQTransportFactoryZMQ: move the config invariant
    initialization to ctor
  * FairMQChannel: add new ctor that creates usable channel
  * FairMQSocket*: close sockets in dtor
  * FairMQTransportFactory*: terminate context in dtor
  * FairMQChannel: add Bind/Connect facades (for explicit control, e.g. timing)

fairmq/nanomsg/FairMQTransportFactoryNN.h

@@ -27,6 +27,7 @@ class FairMQTransportFactoryNN : public FairMQTransportFactory
 {
   public:
     FairMQTransportFactoryNN();
+    ~FairMQTransportFactoryNN() override;
 
     void Initialize(const FairMQProgOptions* config) override;
 
@@ -45,8 +46,6 @@ class FairMQTransportFactoryNN : public FairMQTransportFactory
     void Shutdown() override;
     void Terminate() override;
 
-    ~FairMQTransportFactoryNN() override {};
-
   private:
     static FairMQ::Transport fTransportType;
 };