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/chain.cpp

@@ -82,9 +82,10 @@ CBlockIndex* CBlockIndex::GetAncestor(int height)
     while (heightWalk > height) {
         int heightSkip = GetSkipHeight(heightWalk);
         int heightSkipPrev = GetSkipHeight(heightWalk - 1);
-        if (heightSkip == height ||
-            (heightSkip > height && !(heightSkipPrev < heightSkip - 2 &&
-                                      heightSkipPrev >= height))) {
+        if (pindexWalk->pskip != NULL &&
+            (heightSkip == height ||
+             (heightSkip > height && !(heightSkipPrev < heightSkip - 2 &&
+                                       heightSkipPrev >= height)))) {
             // Only follow pskip if pprev->pskip isn't better than pskip->pprev.
             pindexWalk = pindexWalk->pskip;
             heightWalk = heightSkip;