Make CAddrman::Select_ select buckets, not positions, first
Summary:
The original CAddrMan behaviour (before commit e6b343d880f50d52390c5af8623afa15fcbc65a2) was to pick a uniformly random non-empty bucket, and then pick a random element from that bucket. That commit, which introduced deterministic placement of entries in buckets, changed this to picking a uniformly random non-empty bucket position instead. This commit reverts the original high-level behavior, but in the deterministic placement model.
Backport of core#23140.
Depends on D12339.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, sdulfari
Reviewed By: #bitcoin_abc, sdulfari
Differential Revision: https://reviews.bitcoinabc.org/D12340