applySettings

Re-apply a (possibly partial) pack of settings to the running engine and its sessions. This is the port of session::apply_settings. It copies every override in pack into this engine's live settings, then re-drives the knobs that are wired here: rate limits, connections_limit, proxy_*, per-torrent unchoke slots and connection caps. Knobs read once at construction time (request/choke timeouts) take effect for torrents added afterwards.