Advanced Checkpoint Feature

For initial testing on testnet. Read the introduction and instructions in checkpointsync.cpp carefully first.

When setting up checkpoint master node on testnet, set the following parameters:
checkpointkey=<see PM>
checkpointdepth=0

This sets up the node to be checkpoint master node and on automatic policy with immediate checkpoint. This is the strongest policy for defense against 51% attack. There should be at least two testnet nodes or more, with only one of them set up as checkpoint master node.

Sunny