p2p: Add CConnman::GetCurrentBlockRelayOnlyConns()

Summary:
This is a backport of core#17428 [3/7]
https://github.com/bitcoin/bitcoin/pull/17428/commits/bad16aff490dcf87722fbfe202a869fb24c734e1

Depends on D10508

Test Plan: ninja

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10509