Limit scope of all global std::once_flag

Summary:

once_flag is a helper (as the name might suggest) to execute a callable only once. Thus, the scope of the flag does never need to extend beyond where the callable is called. Typically this is function scope.

Move all the flags to function scope to

simplify code review
avoid mistakes where similarly named flags are accidentally exchanged
avoid polluting the global scope

This is a backport of core#19111

Depends on D9260

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9264