Merge #9951: Wallet database handling abstractions... ...various classes in walletdb.h (Wladimir J. van der Laan) ...private in CWalletDBWrapper (Wladimir J. van der Laan) ...composition not inheritance (Wladimir J. van der Laan) ...to global bitdb environment (Wladimir J. van der Laan) ...wallet: Get rid of fFileBacked (Wladimir J. van der Laan) ...Introduce database handle wrapper (Wladimir J. van der Laan)