MXS-2495 Cleanup GWBUF interface

All GWBUF macros that address a single link in a chain are now
simple wrappers for equivalent gwbuf_link-functions.

Next step is to drop the macros and replace their use with calls
to the functions.

server/core/test/test_buffer.cc

@@ -39,6 +39,12 @@
 #include <maxscale/buffer.hh>
 #include <maxscale/hint.h>
 
+/*< Return the byte at offset byte from the start of the unconsumed portion of the buffer */
+#define GWBUF_DATA_CHAR(b, byte) (GWBUF_LENGTH(b) < ((byte) + 1) ? -1 : *(((char*)(b)->start) + 4))
+
+/*< Check that the data in a buffer has the SQL marker*/
+#define GWBUF_IS_SQL(b) (0x03 == GWBUF_DATA_CHAR(b, 4))
+
 /**
  * Generate predefined test data
  *