Fix the number of column for 'Async connections'.

There are only 3 columns (writing, keep-alive, closing), not 4.

Try to improve the code layout for it to be more readable.
Each <th> is on its own line so keep the corresponding "colspan" <td> fields grouped together.

r1738628 introduced a new column, 'Slot'.
Add an empty cell for it in the last line of the table, in order to fix the layout of the Totals.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1757009 13f79535-47bb-0310-9956-ffa450edef68