Route handlers returning QFuture are broken with pipelining

QAbstractHttpServer is allocating a single QHttpServerRequest object per connection. This works fine as long as the server does not start reading next request from until the current one is done processing. This is probably broken when handlers returning QFuture are used and the client does request pipelining.

If reproducing the bug using Qt, note that QNetworkManager may create up to 6 connections before attempting pipelining.