Fix high gateway CPU usage (#300)

It turns out, when an http endpoint request is pending, and the request
buffer shuts down (if there are no containers running, or the serve is
cancelled, etc), we are not returning from this select, and it is
locking/unlocking indefinitely. Relevant pprof dump:


```
(pprof) top
Showing nodes accounting for 29.61s, 95.89% of 30.88s total
Dropped 301 nodes (cum <= 0.15s)
Showing top 10 nodes out of 47
      flat  flat%   sum%        cum   cum%
     9.03s 29.24% 29.24%      9.04s 29.27%  runtime.lock2
     7.94s 25.71% 54.95%      7.95s 25.74%  runtime.unlock2
     7.62s 24.68% 79.63%     28.65s 92.78%  runtime.selectgo
     1.48s  4.79% 84.42%     10.51s 34.03%  runtime.sellock
     1.03s  3.34% 87.76%      9.40s 30.44%  runtime.selunlock
     0.79s  2.56% 90.32%      0.79s  2.56%  runtime.fastrand (inline)
     0.73s  2.36% 92.68%      0.94s  3.04%  context.(*cancelCtx).Done
     0.39s  1.26% 93.94%     29.98s 97.09%  github.com/beam-cloud/beta9/pkg/abstractions/endpoint.(*RequestBuffer).ForwardRequest
     0.37s  1.20% 95.14%      0.37s  1.20%  runtime/internal/syscall.Syscall6
     0.23s  0.74% 95.89%      8.38s 27.14%  runtime.unlock (inline)
```

pkg/abstractions/endpoint/buffer.go

@@ -106,8 +106,10 @@ func (rb *RequestBuffer) ForwardRequest(ctx echo.Context, payload *types.TaskPay
 	for {
 		select {
 		case <-rb.ctx.Done():
+			return nil
 		case <-done:
 			return nil
+		case <-time.After(100 * time.Millisecond):
 		}
 	}
 }