MXS-862: Make packet sequence handling automatic

Made the packet sequence number handling automatic so that it always uses
the correct one.

All functions now have documentation in them. Cleaned up code and added
comments to GSSAPI code.

server/modules/authenticator/gssapi_auth.h

@@ -39,9 +39,10 @@ enum gssapi_auth_state
 /** Common state tracking structure */
 typedef struct gssapi_auth
 {
-    enum gssapi_auth_state state;
-    uint8_t *principal_name;
-    size_t principal_name_len;
+    enum gssapi_auth_state state; /**< Authentication state*/
+    uint8_t *principal_name;      /**< Principal name */
+    size_t principal_name_len;    /**< Length of the principal name */
+    uint8_t sequence;             /**< The next packet seqence number */
 } gssapi_auth_t;
 
 /** These functions can used for the `create` and `destroy` entry points */