Reduce checkpoints' effect on consensus.

Instead of only checking height to decide whether to disable script checks,
actually check whether a block is an ancestor of a checkpoint, up to which
headers have been validated. This means that we don't have to prevent
accepting a side branch anymore - it will be safe, just less fast to
do.

We still need to prevent being fed a multitude of low-difficulty headers
filling up our memory. The mechanism for that is unchanged for now: once
a checkpoint is reached with headers, no headers chain branching off before
that point are allowed anymore.

src/checkpoints.h

@@ -26,9 +26,6 @@ struct CCheckpointData {
     double fTransactionsPerDay;
 };
 
-//! Returns true if block passes checkpoint checks
-bool CheckBlock(const CCheckpointData& data, int nHeight, const uint256& hash);
-
 //! Return conservative estimate of total number of blocks, 0 if unknown
 int GetTotalBlocksEstimate(const CCheckpointData& data);