Rationale:
- this init function can now open multiple wallets (hence
Wallet->Wallets)
- This is named as the antonym to CloseWallets(), which carries out the
opposite action.
This commit just moves a few function declarations and updates callers.
Function bodies are moved in two followup MOVEONLY commits.
This change is desirable because wallet.h/cpp are monolithic and hard to
navigate, so pulling things out and grouping together pieces of related
functionality should improve the organization.
Another proximate motivation is the wallet process separation work in
https://github.com/bitcoin/bitcoin/pull/10973, where (at least initially)
parameter parsing and fee estimation are still done in the main process rather
than the wallet process, and having functions that run in different processes
scrambled up throughout wallet.cpp is unnecessarily confusing.
