Implement RDM mechanism (Github#85)
<Originally opened by Tomek Mrugalski on Github as issue #85 (closed)>
The rfc3315bis draft defines Replay Detection Mechanism. We need to have this implemented in Kea.
We'll go with the simplest model possible. The RDM will be a global 64bits counter (the same value for all subnets and all clients. This value must be retained between server restarts.