[MEDIUM] http: wait for some flush of the response buffer before a new request

[MEDIUM] http: wait for some flush of the response buffer before a new request

If we accept a new request and that request produces an immediate
response (error, redirect, ...), then we may fail to send it in
case of pipelined requests if the response buffer is full. To avoid
this, we check the availability of at least maxrewrite bytes in the
response buffer before accepting a new pipelined request.