Make it very clear when new versions have large schema updates

[mattab]

We had issues in the past (in 0.5) when we released large schema upgrade causing time outs on some hosts, and we had to offer a lot of support in the forums.

I would like to propose a few steps to make this better in future releases (0.5.5 looks like it might contain such massive schema upgrade)

1. Improve the messaging in the welcome update page and display a big red warning message when a large schema update will happen. This is the case whenever a large table (log_ or archive_) have new fields, new INDEXes, or various other bulk updates. We could have a flag/method in the update class that would be set to true when the update can potentially lead to such issues.

2. We should also display the list of SQL queries that are going to be executed so that the user can execute them manually in case of error.

3. Since 0.5 there is a feature that updates can be ran using the safer command line upgrade mechanism. This should also be highlighted.

[robocoder]

(In [1844]) refs #1129, refs #1151 - refactoring to add getSql() to return array of SQL statements

[mattab]

Note: I created the ticket after reading the post mortem at: http://devzone.zend.com/article/11821-Yes-I-Crashed-the-Site

[robocoder]

Brain dump. Not sure how practical this idea is...

see: http://www.php.net/manual/en/function.exec.php#86329

Idea is: (pseudocode)

if not php cli mode
    ignore_user_abort
    disable max_execution_time

    if exec() is not disabled
        exec('nohup php index.php >/dev/null 2>/dev/null &') // run the update in the background
        redirect to tmp/latest/progress.html // a static file that is overwritten by the updater
    else
        run updater // for each update, echo 'something' to the client and flush output buffers to (try) keep connection alive

else
    run updater // foreach update, write to progress.html

[mattab]

Anthon, I'm not going to test your pseudocode, it should like it might work, but generally if users have the exec() command, they might just want to run the command line updater I think.

let me know what you think of my improvements to the update welcome page; I hope this will help users get through large updates in the future

[mattab]

in [1941](again trac didn't pick up this commit, grr)

Fixes #1151
Clarifying update process for large piwik instances. outputs list of SQL queries that will be executed during updates.
Fixes language selector during update, which was only working after second refresh.