Set initial_routing_sync in InitFeatures

The initial_routing_sync feature is set by peer_handler whenever a full
sync of the network graph is desired. It is not explicitly set when
creating features with InitFeatures::supported().

An upcoming refactor will change supported() to known(), which will
return all features known by the implementation. Thus, the
initial_routing_sync flag will need to be set by default. This commit
makes the behavior change ahead of the refactor.