Fix some asmap issues
Summary:
Here are a few things to improve in the asmap implementation. The first two commits are just code improvements. The last one is a bugfix (the exsting code wouldn't correctly apply ASN lookups to mapped/embedded IPv4 addresses).
Backport of core PR18023.
Depends on D8197.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Subscribers: jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D8198