This way xlog.Reader can be sure that if it looked around in such a
window and did not find a sync, then something is not good with the
stream and it does not need to go beyond that limit looking around.

This is a change of the protocol. But it is early days and existing logs
- that we use in the demo, are all below 1000 lines limit, so they will
continue to be loaded ok.

No direct test for actual Loss Of Sync detection - this functionality is
draft for now and should be improved later. However for no-LOS cases
xlog.Reader is already covered with tests.