Create new wallet databases as directories rather than files

This change should make it easier for users to make complete backups of wallets
because they can now just back up the specified `-wallet=<path>` path directly,
instead of having to back up the specified path as well as the transaction log
directory (for incompletely flushed wallets).

Another advantage of this change is that if two wallets are located in the same
directory, they will now use their own BerkeleyDB environments instead using a
shared environment. Using a shared environment makes it difficult to manage and
back up wallets separately because transaction log files will contain a mix of
data from all wallets in the environment.

src/wallet/wallet.cpp

@@ -45,7 +45,6 @@ OutputType g_address_type = OUTPUT_TYPE_NONE;
 OutputType g_change_type = OUTPUT_TYPE_NONE;
 bool g_wallet_allow_fallback_fee = true; //<! will be defined via chainparams
 
-const char * DEFAULT_WALLET_DAT = "wallet.dat";
 const uint32_t BIP32_HARDENED_KEY_LIMIT = 0x80000000;
 
 /**