Jester with HttpBeast is slow if used with ThreadPool 
The code below would work 10 times faster if compiled with `AsyncHttpServer` instead of `HttpBeast`.

I guess it's because it uses `spawn` and competes with `HttpBeast` for same `ThreadPool`.

Not sure if it's a problem or expected behaviour, feel free to close.

```Nim
import os, threadpool, jester

proc process(data: string): string =
  sleep 100
  "processed " & data

routes:
  get "/":
    let cresult = spawn process("something")
    while true:
      await sleep_async 1
      if cresult.is_ready: break
    resp ^cresult
```

Results with HttpBeast `nim c -r --threads:on play.nim` , 2 req/sec

```Bash
wrk -t2 -c2 -d10s http://localhost:5000
Running 10s test @ http://localhost:5000
  2 threads and 2 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   991.70ms   25.97ms   1.00s    90.00%
    Req/Sec     0.30      0.47     1.00     70.00%
  20 requests in 10.06s, 2.99KB read
Requests/sec:      1.99
Transfer/sec:     304.09B
```

Results with AsyncHttpServer `nim c -r --threads:on play.nim` , 20 req/sec

```Bash
wrk -t2 -c2 -d10s http://localhost:5000
Running 10s test @ http://localhost:5000
  2 threads and 2 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   104.47ms    1.67ms 108.73ms   61.98%
    Req/Sec     9.06      0.83    10.00     81.77%
  192 requests in 10.09s, 18.19KB read
Requests/sec:     19.04
Transfer/sec:      1.80KB
```