Increase size of zmq_msg_t to 64 bytes

Increasing it would have at least two benefits -

* More messages would be 'VSM' messages, so it would reduce allocation
overhead a bit.
* Remove any chance of false sharing of things that are, by design,
pushed by value onto a ypipe_t<msg_t> which is shared between two threads.

The only downside I see is slightly increased memory consumption on memory
constrained applications.

- Full discussion of this rationale is part of issue #1295

src/msg.hpp

@@ -92,7 +92,7 @@ namespace zmq
 
         //  Size in bytes of the largest message that is still copied around
         //  rather than being reference-counted.
-        enum { msg_t_size = 48 };
+        enum { msg_t_size = 64 };
         enum { max_vsm_size = msg_t_size - (8 + sizeof (metadata_t *) + 3) };
 
         //  Shared message buffer. Message data are either allocated in one