Implement MoldUDP64 client state

The MoldUDP64 client is initially in the 'UNKNOWN' state. On the first
packet, it goes to the 'BACKFILL' or 'SYNCHRONIZED' state. Should it
fall out of synchronization with the server, it goes to the 'GAP_FILL'
state until it regains synchronization.
Merge pull request #8 from jvirtanen/features/moldudp64-client-state

Implement MoldUDP64 client state