wallet: CWalletDB CDB composition not inheritance

CWalletDB now contains a CDB instead of inheriting from it.

This makes it easier to replace the internal transaction with a different
database, without leaking through internals.

src/wallet/db.h

@@ -145,10 +145,10 @@ class CDB
     bool fFlushOnClose;
     CDBEnv *env;
 
+public:
     explicit CDB(CWalletDBWrapper& dbw, const char* pszMode = "r+", bool fFlushOnCloseIn=true);
     ~CDB() { Close(); }
 
-public:
     void Flush();
     void Close();
     static bool Recover(const std::string& filename, void *callbackDataIn, bool (*recoverKVcallback)(void* callbackData, CDataStream ssKey, CDataStream ssValue));
@@ -165,7 +165,7 @@ class CDB
     CDB(const CDB&);
     void operator=(const CDB&);
 
-protected:
+public:
     template <typename K, typename T>
     bool Read(const K& key, T& value)
     {