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/protocol/MySQL/MySQLClient/mysql_client.c

@@ -479,6 +479,9 @@ int gw_read_client_event(DCB* dcb)
 static int
 gw_read_do_authentication(DCB *dcb, GWBUF *read_buffer, int nbytes_read)
 {
+    ss_debug(uint8_t hdr[MYSQL_HEADER_LEN]);
+    ss_dassert(gwbuf_copy_data(read_buffer, 0, MYSQL_HEADER_LEN, hdr) == MYSQL_HEADER_LEN &&
+               MYSQL_GET_PACKET_LEN(hdr) + MYSQL_HEADER_LEN == gwbuf_length(read_buffer));
     /** Allocate the shared session structure */
     if (dcb->data == NULL && (dcb->data = mysql_session_alloc()) == NULL)
     {