fees: make the class FeeFilterRounder thread-safe
Summary:
Make the class FeeFilterRounder thread-safe so that its methods can be called concurrently by different threads on the same object. Currently it has just one method (round()). The second commit is optional, but it improves readability, showing that the feeset member will never be changed, thus does not need protection from concurrent access.
Backport of core#24407 and core#26306.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, roqqit
Reviewed By: roqqit
Differential Revision: https://reviews.bitcoinabc.org/D16470