[backport#15638] [build] Move wallet load functions to wallet/load unit

Summary:
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

Depends on D5997

This is a partial backport of Core PR15638

Test Plan:

../configure --enable-deprecated-build-system [--disable-wallet]
make check
./test/functional/test_runner.py

../cmake -GNinja [-DBUILD_BITCOIN_WALLET=OFF]
ninja check-all

Reviewers: #bitcoin_abc, deadalnix, nakihito

Reviewed By: #bitcoin_abc, deadalnix, nakihito

Subscribers: nakihito, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6000