Refactor RPC: Access wallet through GetWalletForJSONRPCRequest

Summary:
Partially backport core's PR8775, namely:

RPC/Wallet: Pass CWallet as pointer to helper functions
RPC: Do all wallet access through new GetWalletForJSONRPCRequest

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: schancel, #bitcoin_abc, jasonbcox

Reviewed By: schancel, #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D925