[#1183] wallet.NewMultiWallet can create SPV wallets for BTC and BCH
[#1183] wallet.NewMultiWallet can create SPV wallets for BTC and BCH

Prior refactor removed this behavior, so this commit restores it
following the newly created pattern.

OpenBazaar/openbazaar-go,OpenBazaar/openbazaar-go,OpenBazaar/openbazaar-go