fixed random blocking of async server

- asynchronous and multi-thread programming is hard - race conditions could be difficult to track
- I spent a lot of time on this issue, which was reproduced using the wrk tool and a lot of concurrent connections for a long time
- it was more easily reproduced on Windows, but could potentially affect all platforms, and may be the culprit of very rare race condition on other platforms like Linux (seen e.g. on TFB)
- in fact, it was a one-liner - as often with race conditions :) - just move one flag assignment at the right place

src/mormot.commit.inc

@@ -1 +1 @@
-'2.1.5811'
+'2.1.5812'

src/net/mormot.net.async.pas

@@ -1869,8 +1869,8 @@ procedure TAsyncConnectionsThread.Execute;
         atpReadPending:
           // secondary threads wait, then read and process pending events
           begin
-            fWaitForReadPending := true;
             fEvent.ResetEvent;
+            fWaitForReadPending := true; // to be set just before WaitForEver
             fEvent.WaitForEver;
             if Terminated then
               break;