Prevent UB in DeleteLock() function

Summary:

• refactor: Add LockStackItem type alias
• refactor: Add LockPair type alias
• refactor: Refactor duplicated code into LockHeld()
• Replace thread_local g_lockstack with a mutex-protected map

This change prevents UB in case of early g_lockstack destroying.

Co-authored-by: Wladimir J. van der Laan <laanwj@protonmail.com>

• doc: Add and fix comments about never destroyed objects

This is a backport of Core PR18881

Test Plan:

ninja all check-all

Also in debug mode.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7525