MXS-2042: fix multistatments hang

[ybbct]

I am contributing the new code of the whole pull request, including one or
several files that are either new files or modified ones, under the BSD-new
license.

If enable multi statements, and all result sets returned in one packet, then it will hang;

For example

select name from t1 where id = 1; select name from t1 where id = 3; select name from t1 where id = 2

Because function modutil_count_signal_packets will return more than 2;

[markus456]

If you have example SQL that causes this, can you open a new bug report on Jira? This does look like it solves a bug.

We can create a test case once you've filed the bug report for this.