[addrman] Set m_asmap in CAddrMan initializer list
Summary:
This allows us to make it const.
Partial backport of core#22791:
https://github.com/bitcoin/bitcoin/pull/22791/commits/f572f2b2048994b3b50f4cfd5de19e40b1acfb22#diff-b1e19192258d83199d8adaa5ac31f067af98f63554bfdd679bd8e8073815e69d
Depends on D12292.
Test Plan:
ninja all check
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D12293