[RFC]: What's wrong with me or with httpclient :-)

[vega]

I don't want to file an issue, because maybe the problem is in my code. Below 
is the minimal test (also available [as 
gist](https://gist.github.com/vegansk/8d4126fa5205f1d86145891b02467f46)).

So, the problem is that if I use deprecated API from httpclient, I get maximum 
performance while producing http request. 
    
    
    TEST[SyncClient]: Processed 1000 requests in 40 seconds!
    TEST[DeprecatedAPI]: Processed 1000 requests in 1 seconds!
    TEST[ApacheBenchmark]: Processed 1000 requests in 1 seconds!
    

And sometimes test that uses deprecated API fails with error: 
    
    
    Traceback (most recent call last)
    test.nim(64)             test
    test.nim(59)             main
    test.nim(37)             runSyncTest
    httpclient.nim(646)      postContent
    httpclient.nim(618)      post
    httpclient.nim(529)      request
    httpclient.nim(482)      request
    net.nim(1507)            connect
    os.nim(154)              raiseOSError
    Error: unhandled exception: Connection refused [OSError]
    

Why `postContent` is so slow when using HttpClient? When I replace 
`postContent` on `get`, both tests works with the same speed, 1 second.
    
    
    import asynchttpserver,
           httpclient,
           asyncdispatch,
           net,
           os,
           osproc,
           times
    
    type
      TestType = enum SyncClient, DeprecatedAPI, ApacheBenchmark
    
    const port = Port(8888)
    const testType = SyncClient
    const requestsTotal = 1000
    const postBody = 
"0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
    
    proc serverThread {.thread.} =
      var server = newAsyncHttpServer()
      proc cb(req: Request) {.async.} =
        await respond(req, Http200, "OK")
      
      waitFor serve(server, port, cb)
    
    proc runSyncTest(requests: int) =
      when testType == SyncClient:
        let c = newHttpClient()
      when testType != ApacheBenchmark:
        for _ in 1..requests:
          when testType == SyncClient:
            discard c.postContent("http://localhost:"; & $port, postBody)
          else:
            discard postContent("http://localhost:"; & $port, postBody)
        when testType == SyncClient:
          c.close
      else:
        let file = getTempDir() / "httpclient.test"
        defer:
          removeFile file
        writeFile file, postBody
        let cmd = "ab -n " & $requests & " -p " & file & " http://localhost:"; & 
$port & "/"
        echo "AB command: ", cmd
        discard execCmd(cmd)
    
    proc main() =
      var server: Thread[void]
      createThread(server, serverThread)
      let t = getTime()
      runSyncTest(requestsTotal)
      
      echo "TEST[", testType, "]: Processed ", requestsTotal, " requests in ", 
getTime() - t, " seconds!"
    
    when isMainModule:
      main()