dbwrapper: Pass parent CDBWrapper into CDBBatch and CDBIterator

Zcash: Makes future extensions easier.

Original description (when this was introduced upstream):

Pass parent wrapper directly instead of obfuscation key. This
makes it possible for other databases which re-use this code
to use other properties from the database.

Add a namespace dbwrapper_private for private functions to be used
only in dbwrapper.h/cpp and dbwrapper_tests.

src/test/dbwrapper_tests.cpp

@@ -57,7 +57,7 @@ BOOST_AUTO_TEST_CASE(dbwrapper_batch)
         uint256 in3 = GetRandHash();
 
         uint256 res;
-        CDBBatch batch;
+        CDBBatch batch(dbw);
 
         batch.Write(key, in);
         batch.Write(key2, in2);