Session is not closed if no changes are done

[kossebau]

Sessions should be closed if the last member leaves. This does not happen currently.

Drawback is that this also means that the whole stack of operations created is kept around as well as the copy of the original document, so using storage in db and filesystem.
Additionally the startup of the session is taking longer and longer by the time, because all ops ever created have to be replayed, due to the current design.

[VicDeo]

Closing fixed except one particular case:
Session cleanup happens only after successful save currently.
When no changes are done to document, the session is not saved (no 'save' request from editor).

We need a separate Ajax request on editor shutdown in case there are no data to save. Or leave it as is - it will store the last cursor position (to go on reading from this place next time)

Updated title accordingly