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/instance/proxy_cidrs_monitor.rs

@@ -1,9 +1,9 @@
 use std::collections::BTreeSet;
 use std::sync::{Arc, Weak};
-use std::time::Instant;
 
 use crate::common::global_ctx::{ArcGlobalCtx, GlobalCtxEvent};
 use crate::peers::peer_manager::PeerManager;
+use hotpath::instant::Instant;
 use tokio_util::task::AbortOnDropHandle;
 
 /// ProxyCidrsMonitor monitors changes in proxy CIDRs from peer routes