Use designated initializers for NodeEvictionCandidate
Summary:
This concludes backport of core#25472 (bypasses core#24531)
Note that we already use designated initializers for mempool_opts in our codebase
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D18241