Moves the following wallet load functions to a new wallet/load unit in
the libbitcoin_wallet library. All other functions in wallet/init remain
in libbitcoin_server:
- `VerifyWallets`
- `LoadWallets`
- `StartWallets`
- `FlushWallets`
- `StopWallets`
- `UnloadWallets`
https://github.com/bitcoin/bitcoin/pull/15638/commits/9eaeb7fb8d4ab0d4493849e6c17e314fd75fea9c
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Depends on D5997
This is a partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/15638 | PR15638]]