[Bitcoin-ABC/bitcoin-abc] walletdir option (#240)

Hello. Sorry for the late response. This option is more about multiwallets support added in 0.16.0 version of Core. I didn’t dig deep into the code just searched for `walletdir` occurrences:

### Docs
https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.16.0.md#wallets-directory-configuration—walletdir

### Changes
https://github.com/bitcoin/bitcoin/blob/master/src/wallet/db.cpp#L319
https://github.com/bitcoin/bitcoin/blob/master/src/wallet/db.cpp#L343

https://github.com/bitcoin/bitcoin/blob/master/src/wallet/rpcwallet.cpp#L3090
https://github.com/bitcoin/bitcoin/blob/master/src/wallet/rpcwallet.cpp#L3143
https://github.com/bitcoin/bitcoin/blob/master/src/wallet/rpcwallet.cpp#L3196

https://github.com/bitcoin/bitcoin/blob/master/src/wallet/wallet.cpp#L3962

### New files
https://github.com/bitcoin/bitcoin/blob/master/src/wallet/init.cpp
https://github.com/bitcoin/bitcoin/blob/master/src/wallet/walletutil.h
https://github.com/bitcoin/bitcoin/blob/master/src/wallet/walletutil.cpp