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/1-sampler-sink/ex1-sampler-sink.json

@@ -6,7 +6,7 @@
             "id": "sampler1",
             "channel":
             {
-                "name": "data-out",
+                "name": "data",
                 "socket":
                 {
                     "type": "push",
@@ -24,7 +24,7 @@
             "id": "sink1",
             "channel":
             {
-                "name": "data-in",
+                "name": "data",
                 "socket":
                 {
                     "type": "pull",