Introduce the pimpl pattern for CAddrMan to separate the implementation details
from the externally used object representation. This reduces compile-time
dependencies and conceptually clarifies AddrMan's interface from the
implementation specifics.
Since the unit & fuzz tests currently rely on accessing CAddrMan internals, this
commit introduces addrman_impl.h, which is exclusively imported by addrman.cpp
and test files.
Review hint: git diff --color-moved=dimmed-zebra
--color-moved-ws=ignore-all-space
