rpc: Add unloadwallet RPC, release notes, and tests

Summary:
This adds the unloadwallet RPC, release notes, and tests for it.

Partial backport of Core PR13111
https://github.com/bitcoin/bitcoin/pull/13111/commits/6608c369b1a6cfc1d5b4a7905c193baa999ba84c
https://github.com/bitcoin/bitcoin/pull/13111/commits/9f9b50d5feb1e604283c463e289e83b63a849a8c
https://github.com/bitcoin/bitcoin/pull/13111/commits/4940a20a46685cd56ea045d8cc7fe058c6222431
https://github.com/bitcoin/bitcoin/pull/13111/commits/ccbf7ae7496fd13b4147aa13d7408712bd90c614
https://github.com/bitcoin/bitcoin/pull/13111/commits/0b82bac76d0f842bd2294a290388536951fbc576 (only the additional test added to wallet_multiwallet.py is taken from here).

Depends on D4252

Note to reviewers: This RPC has a bug discussed here https://github.com/bitcoin/bitcoin/pull/14941

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4253