blockfilter: Use unordered_set instead of set in blockfilter.

Summary:
remaining backport of PR14074:
fef5adcc3 blockfilter: Use unordered_set instead of set in blockfilter.

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T589

Differential Revision: https://reviews.bitcoinabc.org/D2926