Merge #12923: util: Pass pthread_self() to pthread_setschedparam instead of 0

Summary:
b86730a util: Remove designator initializer from ScheduleBatchPriority (Wladimir J. van der Laan)
cff66e6 util: Pass pthread_self() to pthread_setschedparam instead of 0 (Wladimir J. van der Laan)

Pull request description:

Nowhere in the man page of `pthread_setschedparam` it is mentioned that `0` is a valid value. The example uses `pthread_self()`, so should we.

(noticed by Anthony Towns)
Fixes #12915.

Tree-SHA512: 249e93b1ae7e3ba28de6ee6288400b91d21ca1b4ca41d82211f6c9609b62deb5ac87182c7bf08471d3a3e0c1af314c9ecd41f8ae864febe963b1de8a816dd82f

Backport of Core PR12923
https://github.com/bitcoin/bitcoin/pull/12923/

Depends on D3953

Test Plan:

make check
test_runner.py

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

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3954