refactor: Move ScheduleBatchPriority to its own file

Summary:
With the previous move of AlertNotify out of the validation file, and
thus out of the kernel library, ScheduleBatchPriority is the last
remaining function used by the kernel library from util/system. Move it
to its own file, such that util/system can be moved out of the util
library in the following few commits.

Moving util/system out of the kernel library removes further networking
as well as shell related code from it.

This is a partial backport of core#27636
https://github.com/bitcoin/bitcoin/pull/27636/commits/9ec5da36b62276ae22e348f26f88aaf646357d6d

Depends on D16207

Test Plan:
With bitcoin-chainstate
ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16208