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/init.cpp

@@ -162,7 +162,6 @@ public:
     // Writes do not need similar protection, as failure to write is handled by the caller.
 };
 
-static CCoinsViewDB *pcoinsdbview = NULL;
 static CCoinsViewErrorCatcher *pcoinscatcher = NULL;
 static std::unique_ptr<ECCVerifyHandle> globalVerifyHandle;