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/validation.cpp

@@ -175,6 +175,7 @@ CBlockIndex* FindForkInGlobalIndex(const CChain& chain, const CBlockLocator& loc
     return chain.Genesis();
 }
 
+CCoinsViewDB *pcoinsdbview = NULL;
 CCoinsViewCache *pcoinsTip = NULL;
 CBlockTreeDB *pblocktree = NULL;