More thread safety annotation coverage

Summary:

• rpc/blockchain.cpp: Remove g_utxosetscan mutex that is only protecting a single atomic variable
• wallet/wallet.h: Remove mutexScanning which was only protecting a single atomic bool
• net: fMsgProcWake use LOCK instead of lock_guard
• rpc/blockchain.cpp: thread safety annotations for latestblock
• test/checkqueue_tests: thread safety annotations
• util/system.cpp: add thread safety annotations for dir_locks
• logging: thread safety annotations

Adds LockGuard helper in threadsafety.h to replace lock_guard<mutex>
when LOCK(Mutex) isn't available for use.

This is a backport of Core PR16127

Test Plan:

ninja all check

Using clang

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7523