txdb: Add Cursor() method to CCoinsView to iterate over UTXO set

Add a method Cursor() to CCoinsView that returns a cursor which can be
used to iterate over the whole UTXO set.

- rpc: Change gettxoutsetinfo to use new Cursor method

- txdb: Remove GetStats method - Now that GetStats is implemented in
  terms of Cursor, remove it.

src/test/coins_tests.cpp

@@ -61,8 +61,6 @@ public:
             hashBestBlock_ = hashBlock;
         return true;
     }
-
-    bool GetStats(CCoinsStats& stats) const { return false; }
 };
 
 class CCoinsViewCacheTest : public CCoinsViewCache