Add mapSerials infrastructure to CCoinsView.

This adds the TXDB/CCoinsViewCache primitives necessary for
writing consensus rules for mapSerials later.

src/txdb.h

@@ -37,6 +37,7 @@ public:
     CCoinsViewDB(size_t nCacheSize, bool fMemory = false, bool fWipe = false);
 
     bool GetAnchorAt(const uint256 &rt, libzerocash::IncrementalMerkleTree &tree) const;
+    bool GetSerial(const uint256 &serial) const;
     bool GetCoins(const uint256 &txid, CCoins &coins) const;
     bool HaveCoins(const uint256 &txid) const;
     uint256 GetBestBlock() const;
@@ -44,7 +45,8 @@ public:
     bool BatchWrite(CCoinsMap &mapCoins,
                     const uint256 &hashBlock,
                     const uint256 &hashAnchor,
-                    CAnchorsMap &mapAnchors);
+                    CAnchorsMap &mapAnchors,
+                    CSerialsMap &mapSerials);
     bool GetStats(CCoinsStats &stats) const;
 };