Add rewind logic to deal with post-fork software updates

Includes logic for dealing with pruning by Suhas Daftuar.

src/chain.h

@@ -94,6 +94,8 @@ enum BlockStatus: uint32_t {
     BLOCK_FAILED_VALID       =   32, //! stage after last reached validness failed
     BLOCK_FAILED_CHILD       =   64, //! descends from failed block
     BLOCK_FAILED_MASK        =   BLOCK_FAILED_VALID | BLOCK_FAILED_CHILD,
+
+    BLOCK_OPT_WITNESS       =   128, //! block data in blk*.data was received with a witness-enforcing client
 };
 
 /** The block chain is a tree shaped structure starting with the