lib/pool: fix unreliable TestPoolMaxBufferMemory test

This turned out to be a problem in the tests. The tests used to do

1. allocate
2. increment
3. free
4. decrement

But if one goroutine had just completed 2 and another had just
completed 3 then this can cause the test to register too many
allocations.

This was fixed by doing the test in this order instead:

1. allocate
2. increment
3. decrement
4. free

The 4 operations are atomic.

Fixes #8813

lib/pool/pool_test.go

@@ -288,22 +288,24 @@ func TestPoolMaxBufferMemory(t *testing.T) {
 			}
 		}
 	)
-	for i := 0; i < 20; i++ {
+	const trials = 50
+	for i := range trials {
 		wg.Add(1)
 		go func() {
 			defer wg.Done()
-			if i < 4 {
-				buf := bp.GetN(i + 1)
-				countBuf(i + 1)
-				time.Sleep(100 * time.Millisecond)
+			if i < trials/2 {
+				n := i%4 + 1
+				buf := bp.GetN(n)
+				countBuf(n)
+				time.Sleep(1 * time.Millisecond)
+				countBuf(-n)
 				bp.PutN(buf)
-				countBuf(-(i + 1))
 			} else {
 				buf := bp.Get()
 				countBuf(1)
-				time.Sleep(100 * time.Millisecond)
-				bp.Put(buf)
+				time.Sleep(1 * time.Millisecond)
 				countBuf(-1)
+				bp.Put(buf)
 			}
 		}()
 	}