NLSR

The sync system depends on the specific implementation of the LSDB for understanding if an LSA is new. This works for now, but makes the system unfriendly to change, because a change in either the SyncLogicHandler or the Lsdb classes requires a change in the other.

This is not always avoidable, but can be mitigated if the sync system defines a function interface for determining LSA newness, and it consuts such a function in the form of a constructor-time argument.

This makes it easier to vary the LSDB, because the classes are decoupled by a function call. It makes it easier to vary the sync system, because its functionality now depends on an "abstract" function.