Use WalletManager as in bitcoin-core

[scravy]

I am porting the wallet manager which has recently been proposed in bitcoin/bitcoin#13034 for bitcoin-core. Wallet Manager was actually part of bitcoin before, removed, now makes it back. I find it very useful and makes the code clear, because right now wallets are "just there".

[scravy]

The feature seems to be highly controversial (looking at the PR). I say we just keep in line with what bitcoin does and we'll get this anyway when we merge with the next version