[Standards] mobile optimizations

[Alexander Gnauck]

Fabio Forno wrote:
> I like this one, since it always has a backup when no sinchronization
> is needed. Moreover the server can store only a window of changes, and
> send the whole roster if the last known by the client is too old

+1, I don't think we want to keep all roster revisions from the 
beginning. If servers will give us such a option this is great, but I 
don't think it should be required.
For "normal" usage about 100 revisions should be fine.

Alex