perf(monitors): remove std::function heap allocs and use direct map find in pub-sub fanout

[archief2910]

Description

ActiveMonitors::process runs for every new event that is applied to live subscriptions. It previously looked up candidate monitor sets with upper_bound + prev + a while loop and a std::function<bool(const T&)> built from a lambda for each of id, author, each tag, and kind.

Monitor indexes are installed with one btree_map entry per exact key (event id, author pubkey, tag spec string, kind). Keys are unique, and every call site only ever used an equality predicate (id == val, etc.). For such a map, the only matching node is m.find(key); the old walk could touch at most that same entry.

This change replaces that path with a small templated lookupMonitors helper that calls find and processes the MonitorSet when present. It removes per-event std::function construction (and associated type erasure / allocation behavior) and simplifies the lookup logic with no change to which monitors are considered.

Files: src/ActiveMonitors.h

Motivation and context

Live subscription fanout is on the hot path when the relay is under write load. Fewer temporary objects and a direct map lookup improve CPU and allocator pressure under realistic workloads, without changing Nostr semantics.

How has this been tested?

• make -j4 (full tree)
• Existing relay / subscription behavior unchanged by construction (same keys, same processMonitorSet body); no new tests added.
• script automated testing

Types of changes

• Non-functional change (docs, style, minor refactor)
• Bug fix
• New feature / improvement (performance)
• Breaking change

Checklist

• Matches project style
• Build passes locally

[hoytech]

Nice one, thanks! I think this is a vestigial feature for when we supported prefix lookup.