Back to ct_sync

I've managed to get back to work on ct_sync again. The final steps towards full multi-master operation are underway. Apart from some changes to the protocol on the wire, there is a major reorganization of almost all involved data structures.

I'm deeply sorry for not having been able to continue at the pace that I wanted (and promised some customers), but there have been lots of issues that I couldn't push back and had to deal with them immediately.