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/txdb.cpp

@@ -310,6 +310,7 @@ bool CBlockTreeDB::LoadBlockIndexGuts()
                 pindexNew->nSolution      = diskindex.nSolution;
                 pindexNew->nStatus        = diskindex.nStatus;
                 pindexNew->nTx            = diskindex.nTx;
+                pindexNew->nSproutValue   = diskindex.nSproutValue;
 
                 if (!CheckProofOfWork(pindexNew->GetBlockHash(), pindexNew->nBits, Params().GetConsensus()))
                     return error("LoadBlockIndex(): CheckProofOfWork failed: %s", pindexNew->ToString());