p2p: Add DumpAnchors()

Summary:

This is an implementation of #17326:

• all (currently 2) outbound block-relay-only connections are dumped to anchors.dat file
• on restart a node tries to connect to the addresses from anchors.dat

This PR prevents a type of eclipse attack when an attacker exploits a victim node restart to force it to connect to new, probably adversarial, peers.

This is a backport of core#17428 [1/7]
https://github.com/bitcoin/bitcoin/pull/17428/commits/567008d2a0c95bd972f4031f31647c493d1bc2e8

Test Plan: ninja

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10507