fix(pool): pool performance (#3565)

* perf(pool): replace hookManager RWMutex with atomic.Pointer and add predefined state slices

- Replace hookManager RWMutex with atomic.Pointer for lock-free reads in hot paths
- Add predefined state slices to avoid allocations (validFromInUse, validFromCreatedOrIdle, etc.)
- Add Clone() method to PoolHookManager for atomic updates
- Update AddPoolHook/RemovePoolHook to use copy-on-write pattern
- Update all hookManager access points to use atomic Load()

Performance improvements:
- Eliminates RWMutex contention in Get/Put/Remove hot paths
- Reduces allocations by reusing predefined state slices
- Lock-free reads allow better CPU cache utilization

* perf(pool): eliminate mutex overhead in state machine hot path

The state machine was calling notifyWaiters() on EVERY Get/Put operation,
which acquired a mutex even when no waiters were present (the common case).

Fix: Use atomic waiterCount to check for waiters BEFORE acquiring mutex.
This eliminates mutex contention in the hot path (Get/Put operations).

Implementation:
- Added atomic.Int32 waiterCount field to ConnStateMachine
- Increment when adding waiter, decrement when removing
- Check waiterCount atomically before acquiring mutex in notifyWaiters()

Performance impact:
- Before: mutex lock/unlock on every Get/Put (even with no waiters)
- After: lock-free atomic check, only acquire mutex if waiters exist
- Expected improvement: ~30-50% for Get/Put operations

* perf(pool): use predefined state slices to eliminate allocations in hot path

The pool was creating new slice literals on EVERY Get/Put operation:
- popIdle(): []ConnState{StateCreated, StateIdle}
- putConn(): []ConnState{StateInUse}
- CompareAndSwapUsed(): []ConnState{StateIdle} and []ConnState{StateInUse}
- MarkUnusableForHandoff(): []ConnState{StateInUse, StateIdle, StateCreated}

These allocations were happening millions of times per second in the hot path.

Fix: Use predefined global slices defined in conn_state.go:
- validFromInUse
- validFromCreatedOrIdle
- validFromCreatedInUseOrIdle

Performance impact:
- Before: 4 slice allocations per Get/Put cycle
- After: 0 allocations (use predefined slices)
- Expected improvement: ~30-40% reduction in allocations and GC pressure

* perf(pool): optimize TryTransition to reduce atomic operations

Further optimize the hot path by:
1. Remove redundant GetState() call in the loop
2. Only check waiterCount after successful CAS (not before loop)
3. Inline the waiterCount check to avoid notifyWaiters() call overhead

This reduces atomic operations from 4-5 per Get/Put to 2-3:
- Before: GetState() + CAS + waiterCount.Load() + notifyWaiters mutex check
- After: CAS + waiterCount.Load() (only if CAS succeeds)

Performance impact:
- Eliminates 1-2 atomic operations per Get/Put
- Expected improvement: ~10-15% for Get/Put operations

* perf(pool): add fast path for Get/Put to match master performance

Introduced TryTransitionFast() for the hot path (Get/Put operations):
- Single CAS operation (same as master's atomic bool)
- No waiter notification overhead
- No loop through valid states
- No error allocation

Hot path flow:
1. popIdle(): Try IDLE → IN_USE (fast), fallback to CREATED → IN_USE
2. putConn(): Try IN_USE → IDLE (fast)

This matches master's performance while preserving state machine for:
- Background operations (handoff/reauth use UNUSABLE state)
- State validation (TryTransition still available)
- Waiter notification (AwaitAndTransition for blocking)

Performance comparison per Get/Put cycle:
- Master: 2 atomic CAS operations
- State machine (before): 5 atomic operations (2.5x slower)
- State machine (after): 2 atomic CAS operations (same as master!)

Expected improvement: Restore to baseline ~11,373 ops/sec

* combine cas

* fix linter

* try faster approach

* fast semaphore

* better inlining for hot path

* fix linter issues

* use new semaphore in auth as well

* linter should be happy now

* add comments

* Update internal/pool/conn_state.go

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* address comment

* slight reordering

* try to cache time if for non-critical calculation

* fix wrong benchmark

* add concurrent test

* fix benchmark report

* add additional expect to check output

* comment and variable rename

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

internal/proto/peek_push_notification_test.go

@@ -370,9 +370,17 @@ func BenchmarkPeekPushNotificationName(b *testing.B) {
 			buf := createValidPushNotification(tc.notification, "data")
 			data := buf.Bytes()
 
+			// Reuse both bytes.Reader and proto.Reader to avoid allocations
+			bytesReader := bytes.NewReader(data)
+			reader := NewReader(bytesReader)
+
 			b.ResetTimer()
+			b.ReportAllocs()
 			for i := 0; i < b.N; i++ {
-				reader := NewReader(bytes.NewReader(data))
+				// Reset the bytes.Reader to the beginning without allocating
+				bytesReader.Reset(data)
+				// Reset the proto.Reader to reuse the bufio buffer
+				reader.Reset(bytesReader)
 				_, err := reader.PeekPushNotificationName()
 				if err != nil {
 					b.Errorf("PeekPushNotificationName should not error: %v", err)