FairMQ: Extend Multipart and messaging API

- Extend the multipart API to allow sending vectors of messages or helper
   thin wrapper FairMQParts. See example in examples/MQ/8-multipart.
 - NewMessage() can be used in devices instead of
   fTransportFactory->CreateMessage().
   Possible arguments remain unchanged (no args, size or data+size).
 - Send()/Receive() methods can be used in devices instead of
   fChannels.at("chan").at(i).Send()/Receive():
   Send(msg, "chan", i = 0), Receive(msg, "chan", i = 0).
 - Use the new methods in MQ examples and tests.
 - No breaking changes, but FAIRMQ_INTERFACE_VERSION is incremented to 3
   to allow to check for new methods.

examples/MQ/2-sampler-processor-sink/FairMQExample2Sink.cxx

@@ -28,9 +28,9 @@ void FairMQExample2Sink::Run()
 {
     while (CheckCurrentState(RUNNING))
     {
-        unique_ptr<FairMQMessage> msg(fTransportFactory->CreateMessage());
+        unique_ptr<FairMQMessage> msg(NewMessage());
 
-        if (fChannels.at("data-in").at(0).Receive(msg) >= 0)
+        if (Receive(msg, "data2") >= 0)
         {
             LOG(INFO) << "Received message: \""
                       << string(static_cast<char*>(msg->GetData()), msg->GetSize())