Massive SQL stuff

There is a new table called "thread". This table contains several data about the top post of every thread. This helps in increasing SQL speed a lot (since SQL queries like "WHERE id=parent" really don't scale.

Additionally I removed all "LIMIT 1" at queries that does a "DELETE" or "UPDATE", since that leads to massive warnings in the SQL server log file. They aren't useful at these queries.

Then there were many "LEFT JOIN" that should have been "INNER JOIN". This is changed as well.

There is some minor stuff in the pull request as well:
- "vier" has changed a little bit (again)
- the "unset" bug that let to blank profile pages when not being logged in, is solved
- The parsing of pages (parse_url) is improved 
- When Diaspora posts are arriving, then the first url is fetched for further information (like Diaspora is doing it by itself, when a post arrives there)
- The auto completion of OStatus posts can now be deactivated completely
- The queue script now has a checking like poller or cron so that it can only run once at a time