Track net value entering and exiting the Sprout circuit

Delta values will be stored for new blocks; old blocks can be filled in by
re-indexing. The net value currently in the Sprout circuit is only calculated
when delta values for all previous blocks are present.

src/chain.h

@@ -16,6 +16,8 @@
 
 #include <boost/foreach.hpp>
 
+static const int SPROUT_VALUE_VERSION = 1001400;
+
 struct CDiskBlockPos
 {
     int nFile;
@@ -144,6 +146,15 @@ public:
     //! (memory only) The anchor for the tree state up to the end of this block
     uint256 hashAnchorEnd;
 
+    //! Change in value held by the Sprout circuit over this block.
+    //! Will be boost::none for older blocks on old nodes until a reindex has taken place.
+    boost::optional<CAmount> nSproutValue;
+
+    //! (memory only) Total value held by the Sprout circuit up to and including this block.
+    //! Will be boost::none for on old nodes until a reindex has taken place.
+    //! Will be boost::none if nChainTx is zero.
+    boost::optional<CAmount> nChainSproutValue;
+
     //! block header
     int nVersion;
     uint256 hashMerkleRoot;
@@ -172,6 +183,8 @@ public:
         hashAnchor = uint256();
         hashAnchorEnd = uint256();
         nSequenceId = 0;
+        nSproutValue = boost::none;
+        nChainSproutValue = boost::none;
 
         nVersion       = 0;
         hashMerkleRoot = uint256();
@@ -339,6 +352,14 @@ public:
         READWRITE(nBits);
         READWRITE(nNonce);
         READWRITE(nSolution);
+
+        // Only read/write nSproutValue if the client version used to create
+        // this index was storing them.
+        // TODO: See if we can get away with not serializing the boost::optional
+        // one-byte header, without requiring users to reindex on upgrade.
+        if (nType & SER_DISK && nVersion >= SPROUT_VALUE_VERSION) {
+            READWRITE(nSproutValue);
+        }
     }
 
     uint256 GetBlockHash() const