Increase the ip address relay branching factor for unreachable networks

Summary:
Onion addresses propagate very badly among the IPv4/IPv6 network, resulting
in difficulty for those to find each other.

The branching factor 1 is probably so low that propagations die out before
they reach another onion peer. Increase it to 1.5 on average.

This is a backport of core#19728

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10182