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.

make check
./test/functional/test_runner.py wallet_* feature_config_args