Differential Synchronization 

This is a rebase version of the ```differential-synchronization``` branch.

I've removed work that was related to the old restsync server and also squashed some commit as there were a lot of them. 
Would be nice to be able to use a released version of our Android bits, like

https://github.com/matzew/diff-sync-poc/commit/60e4fcf6245e27c53bc6a0178be0497ad071048b