fix(strelaysrv): make the session limiter session-dependent (fixes #10072) (#10073)

### Purpose

Make the session limiter only apply to current session.

### Testing

Relay 2 or more sessions and check if the sum of the connection speed
can exceed the specified per-session rate.

2 sessions (-global-rate=50000000 and -per-session-rate=6250000):


![图片](https://github.com/user-attachments/assets/133e531a-ed49-4890-aef7-821c628bcfc8)

1 session (-global-rate=50000000 and -per-session-rate=6250000):


![图片](https://github.com/user-attachments/assets/ac89ea53-2d8e-4347-9bbc-4780d85e38d7)

cmd/strelaysrv/main.go

@@ -51,7 +51,6 @@ var (
 	globalLimitBps    int
 	overLimit         atomic.Bool
 	descriptorLimit   int64
-	sessionLimiter    *rate.Limiter
 	globalLimiter     *rate.Limiter
 	networkBufferSize int
 
@@ -228,9 +227,6 @@ func main() {
 		}
 	}
 
-	if sessionLimitBps > 0 {
-		sessionLimiter = rate.NewLimiter(rate.Limit(sessionLimitBps), 2*sessionLimitBps)
-	}
 	if globalLimitBps > 0 {
 		globalLimiter = rate.NewLimiter(rate.Limit(globalLimitBps), 2*globalLimitBps)
 	}