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