[db] Create separate database for txindex.

The new TxIndexDB class will be used by a future commit in this
change set.

src/dbwrapper.h

@@ -224,6 +224,9 @@ public:
     CDBWrapper(const fs::path& path, size_t nCacheSize, bool fMemory = false, bool fWipe = false, bool obfuscate = false);
     ~CDBWrapper();
 
+    CDBWrapper(const CDBWrapper&) = delete;
+    CDBWrapper& operator=(const CDBWrapper&) = delete;
+
     template <typename K, typename V>
     bool Read(const K& key, V& value) const
     {