Add rewind logic to deal with post-fork software updates

Includes logic for dealing with pruning by Suhas Daftuar.

src/main.h

@@ -436,6 +436,9 @@ bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state, CBloc
 
 
 
+/** When there are blocks in the active chain with missing data, rewind the chainstate and remove them from the block index */
+bool RewindBlockIndex(const CChainParams& params);
+
 class CBlockFileInfo
 {
 public: