Protect last outbound HB compact block peer

Summary:
If all our high-bandwidth compact block serving peers (BIP 152) stall block
download, then we can be denied a block for (potentially) a long time. As
inbound connections are much more likely to be adversarial than outbound
connections, mitigate this risk by never removing our last outbound HB peer if
it would be replaced by an inbound.

This is a backport of core#22147

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13341