[addrman] Make m_asmap private

Summary:

Add a GetAsmap() getter function that returns a reference to const.

Partial backport of core#22791:
https://github.com/bitcoin/bitcoin/pull/22791/commits/5840476714ffebb2599999c85a23b52ebcff6090

Depends on D12293.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12294