Close session faster 

An explicit close() method on the session has been introduced.
Anywhere in the code following line will close the session properly:

```php
\OC::$session->close();
```

It can be seen as a replacement for php's native function session_write_close().

Using this new function the session is already closed in an earlier execution state on all ajax file operations as well as on all webdav operations.

@karlitschek @bantu @icewind1991 please review
@guruz @dragotin @danimo please use this branch for parallel up- and download tests using mirall