Allocate shared buffer and its data in one chunk

The GWBUF shared buffer and its data is now allocated in one
chunk so that the data directly follows the shared buffer.
That way, creating a GWBUF will involve 2 and not 3 calls to
malloc and freeing one will involve 2 and not 3 calls to free.

include/maxscale/buffer.h

@@ -105,10 +105,10 @@ struct buffer_object_st
  */
 typedef struct
 {
-    unsigned char   *data;     /*< Physical memory that was allocated */
-    int              refcount; /*< Reference count on the buffer */
+    int32_t          refcount; /*< Reference count on the buffer */
     buffer_object_t *bufobj;   /*< List of objects referred to by GWBUF */
     uint32_t         info;     /*< Info bits */
+    unsigned char    data[1];  /*< Actual memory that was allocated */
 } SHARED_BUF;
 
 /**