QNetworkAccessManager: add public methods to pre-TCP/pre-SSL-connect

If an app knows it needs to connect to a host beforehand, it can "warm up" the
connection cache by making DNS lookup, TCP (and if needed SSL) handshake before
the actual HTTP request is sent. When the HTTP request is made, it will be
considerably faster when there is already a working connection.

Here are some typical results from the benchmark:

* Linux desktop with Ethernet:
  "http://www.google.com" full request: 279 ms, pre-connect request: 61 ms,
    difference: 218 ms
  "https://www.google.com" full request: 344 ms, pre-connect request: 60 ms,
    difference: 284 ms

* mobile device (BlackBerry 10) with Wifi:
  "https://www.google.com" full request: 898 ms, pre-connect request: 159 ms,
    difference: 739 ms
  "http://www.google.com" full request: 707 ms, pre-connect request: 200 ms,
    difference: 507 ms

Task-number: QTBUG-30771
Change-Id: I3566b7f08216ab93a39e2024ae7d1ceb7ae21891
Reviewed-by: Jonas Gastal <gastal@intel.com>
Reviewed-by: Richard J. Moore <rich@kde.org>