Merge #10767: [wallet] Clarify wallet initialization / destruction interface

Merge #10767: [wallet] Clarify wallet initialization / destruction interface

5d2a3995e [trivial] fixup comment for VerifyWallets() (John Newbery)
43b0e81d0 [wallet] Add StartWallets() function to wallet/init.cpp (John Newbery)
290f3c56d [wallet] Add RegisterWalletRPC() function to wallet/init.cpp (John Newbery)
062d63102 [wallet] Add CloseWallets() function to wallet/init.cpp (John Newbery)
77fe07c15 [wallet] Add StopWallets() function to wallet/init.cpp (John Newbery)
2da5eafa4 [wallet] Add FlushWallets() function to wallet/init.cpp (John Newbery)
1b9cee66e [wallet] Rename WalletVerify() to VerifyWallets() (John Newbery)
9c76ba18c [wallet] Rename InitLoadWallet() to OpenWallets() (John Newbery)

Pull request description:

  Apologies for the mostly code move only PR. This is a pre-req for both #10740 and #10762

  All wallet component initialization/destruction functions are now in their own `wallet/init.cpp` translation unit and are no longer static functions on the CWallet class. The bitcoin_server also no longer has any knowledge that there are multiple wallets in vpwallet.

  There should be no changes in behavior from this PR.

Tree-SHA512: 7c260eb094f2fa1a88d803769ba60935810968a7309f731135e4b17623b97f18c03bbcd293c942093d1efce62c6c978f9ff484d54dc9a60bc2fcb5af2d160fcd