HA+MT Packet Parking Improvements

This is the third ticket in the HA+MT effort. It covers the implementation of the packet parking improvements described in the HA+MT design:

Initially this ticket called allowing to self-parking the packets. Subsequent analysis has provided a cleaner solution "Core Proactive Parking".

The scope of this ticket will include changes to src/lib/hooks parking mechanics and kea-dhcpX core code. The resultant code should work seamlessly with existing configuration and hooks (after recompilation).