Don't return stale data from CCoinsViewCache::Cursor()

CCoinsViewCache doesn't actually support cursor iteration returning the
current contents of the cache, so raise an error when the cursor method is
called instead of returning a cursor that iterates over stale data.

Also update the gettxoutsetinfo RPC which was relying on the old behavior to be
explicit about which view it is returning data about.

src/coins.h

@@ -212,6 +212,9 @@ public:
     uint256 GetBestBlock() const;
     void SetBestBlock(const uint256 &hashBlock);
     bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock);
+    CCoinsViewCursor* Cursor() const {
+        throw std::logic_error("CCoinsViewCache cursor iteration not supported.");
+    }
 
     /**
      * Check if we have the given utxo already loaded in this cache.