HTTP internals: do not open too many sockets when preconnecting

Each pair of (normal request, preconnect request) requires only one
socket. E.g. if there is 1 preconnect request in-flight and 2 normal
requests, we need only 2 sockets in total, and not 3.

Therefore, we need to keep track of whether a request is
preconnecting or a normal one.

Task-number: QTBUG-31594
Change-Id: If92ccc35abadfa6090d64ee92bd466615909c94c
Reviewed-by: Richard J. Moore <rich@kde.org>
HTTP internals: fix QHttpNetworkRequestPrivate::operator==

We were not comparing all fields.

Spotted-by: Richard J. Moore <rich@kde.org>

Change-Id: I5c1a773b00b64a26606c47ced292808696c9b4bb
Reviewed-by: Richard J. Moore <rich@kde.org>