addrman: improve performance by using more suitable containers

Summary:

CAddrMan uses std::map internally even though it does not require
that the map's elements are sorted. std::map's access time is
O(log(map size)). std::unordered_map is more suitable as it has a
O(1) access time.

Backport of core#18722.

Test Plan:

ninja all check-all

Build and run the unit tests with DEBUG_ADDRMAN defined.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12252