Filter out content in sync that is currently unhandled (#198)

I had a look at sync.cpp and checked which parts of the sync response
are currently handled and which not. As I think it is unnecessary to let the 
unhandled data be transmitted without being handled I added these filters.

In the same term I increased the timeout server-side to 30s as Riot
defaults to this value as well. Especially now when a lots of presence-updates 
are not send anymore this value is more relevant.

It is now also possible to use a filter that is defined in`client/sync_filter`.
Advanced users might want to set an own filter here.

[ci skip]