When rewinding, remove insufficiently-validated blocks

If a block is insufficiently-validated against a particular branch ID, then we
cannot guarantee that even the block header will be valid under the actual
consensus rules the node will want to apply. Instead require that the blocks are
completely re-validated, by removing them from the block index (which is
equivalent to reducing their validity to BLOCK_VALID_UNKNOWN).

src/txdb.h

@@ -59,6 +59,7 @@ private:
     void operator=(const CBlockTreeDB&);
 public:
     bool WriteBatchSync(const std::vector<std::pair<int, const CBlockFileInfo*> >& fileInfo, int nLastFile, const std::vector<const CBlockIndex*>& blockinfo);
+    bool EraseBatchSync(const std::vector<const CBlockIndex*>& blockinfo);
     bool ReadBlockFileInfo(int nFile, CBlockFileInfo &fileinfo);
     bool ReadLastBlockFile(int &nFile);
     bool WriteReindexing(bool fReindex);