multi_index: fix compilation failure with boost >= 1.91
btcsignals: use a single shared_ptr for liveness and callback
signals: remove boost::signals2 from depends and vcpkg
signals: remove boost includes where possible
signals: re-add forward-declares to interface headers
Revert "signals: Temporarily add boost headers to bitcoind and bitcoin-node builds"
signals: remove boost compatibility guards
signals: Add a simplified boost-compatible implementation