perf(core): use quanta::Instant for hot-path timing (#2384)

Replace std::time::Instant with quanta::Instant on per-packet, per-RPC,
and per-session paths. TSC-based, ~5ns vs ~25ns per now() call.

Reuses the existing `extern crate self as hotpath` alias so
`use hotpath::instant::Instant;` resolves to the same quanta type with
or without the hotpath feature. Leaves tokio::time::Instant and
smoltcp::time::Instant untouched.

easytier/src/peers/relay_peer_map.rs

@@ -1,6 +1,7 @@
-use std::{sync::Arc, time::Instant};
+use std::sync::Arc;
 
 use dashmap::DashMap;
+use hotpath::instant::Instant;
 use prost::Message;
 use snow::params::NoiseParams;
 use tokio::sync::{Mutex, OwnedMutexGuard, oneshot};