[development] Drop MySQL 3 support

[Karoly Negyesi]

>> There will not be any security updates for 3.23 from MySQL AB. Enough
>> reason to drop it.
>
> Big +1, and ISP's and Control Panel developers will drop it as well if
> they don't have upstream support to cover their liabilities.
>

While it's quite OK to drop MySQL 3.x support (even Debian stable has 4.0)  
but dropping 4.0 is absolutely out of question. Despite even Debian stable  
has the mysql-server-4.1 package and all, as someone already mentioned  
it's quite a big jump that not everyone will make.

There are few features of MySQL we could utilize but well, at least  
http://drupal.org/node/38297 could be done, we need to move (part of)  
pager_query to database.??sql.inc . The relatively small number of new  
features also indicate that the big change is between 4.1 and 4.0 and not  
between 3.x and 4.0.

The bigger advantage will be somewhat 'behind the curtains' -- we can rely  
on the query cache somewhat more.