Using "term" table to improve performance, OStatus autocomplete, some more performance stuff

Attention!

This pull request contains a huge update job in the update.php. I guess that it can run for 5-15 minutes - depending upon the size of the item table!

I did the following stuff:
- I'm now using the "term" table to store tags there. This speeds up searching for them. Searching can be done better - but in my tests it is better than before.
- Threads from OStatus are now completed. That means that you now can follow the conversation even when you only added one person in that thread.
- The SQL for the community is speed up. I changed the query so that it only shows posts from people who are really on this server. The query that was used before sometimes showed posts from foreign contacts - what I'm considering as a bug.
- In the API I beautified the output of repeated items.
- For performance testing I added some code. It can be used by activating the "rendertime" addon