Fix EOF packet calculation for large rows

The EOF packet calculation function in modutil.cc didn't handle the case
where the payload exceeded maximum packet size and could mistake binary
data for a ERR packet.

The state of a multi-packet payload is now exposed by the
modutil_count_signal_packets function. This allows proper handling of
large multi-packet payloads.

Added minor improvements to mxs1110_16mb to handle testing of this change.

include/maxscale/modutil.h

@@ -59,13 +59,19 @@ GWBUF*          modutil_create_mysql_err_msg(int             packet_number,
  * whole packets. If partial packets are in the buffer, they are ignored.
  * The caller must handle the detection of partial packets in buffers.
  *
+ * On the first invocation, the value pointed by @c skip should be set to false.
+ * On all subsequent calls, for partial result sets, the function uses it to
+ * store the internal state. When the value pointed by @c skip is set to true,
+ * the next call must be done with only unprocessed packets in @c reply.
+ *
  * @param reply      Buffer to use
  * @param n_found    Number of previous found packets
  * @param more       Set to true of more results exist
+ * @param skip       Internal state of the function used for handling large payloads.
  *
  * @return Total number of EOF and ERR packets including the ones already found
  */
-int modutil_count_signal_packets(GWBUF *reply, int n_found, bool* more);
+int modutil_count_signal_packets(GWBUF *reply, int n_found, bool* more, bool* skip);
 
 mxs_pcre2_result_t modutil_mysql_wildcard_match(const char* pattern, const char* string);