MXS-852: Store the internal ID in the buffer

If the internal ID is stored in the buffer when it is moving inside the
readwritesplit router, the RWBackend can manage the execution of all
commands with a statement ID by replacing the stored ID with the correct
value.

include/maxscale/buffer.h

@@ -212,6 +212,21 @@ extern void gwbuf_free(GWBUF *buf);
  */
 extern GWBUF *gwbuf_clone(GWBUF *buf);
 
+/**
+ * @brief Deep clone a GWBUF
+ *
+ * Clone the data inside a GWBUF into a new buffer. The created buffer has its
+ * own internal buffer and any modifications to the deep cloned buffer will not
+ * reflect on the original one. Any buffer objects attached to the original buffer
+ * will not be copied. Only the buffer type of the original buffer will be copied
+ * over to the cloned buffer.
+ *
+ * @param buf Buffer to clone
+ *
+ * @return Deep copy of @c buf or NULL on error
+ */
+extern GWBUF* gwbuf_deep_clone(const GWBUF* buf);
+
 /**
  * Compare two GWBUFs. Two GWBUFs are considered identical if their
  * content is identical, irrespective of whether one is segmented and