Session allows to reuse SessionRequests to the same host by making them
keeping alive connection. Turns out, though, that libcurl already does
that for us, and we didn't gain any speedup we thought we'd get.
Usage:
```
Networking::Session s;
Networking::SessionRequest *request = s.get(url);
request->startAndWait();
warning("HTTP GET: %s", request->text());
s.close();
```
You can still use SessionRequest without Session (but you can't put them
on stack!):
```
Networking::SessionRequest *request = new
Networking::SessionRequest(url);
request->startAndWait();
warning("HTTP GET: %s", request->text());
request->close();
```
It is to be used in a Session, though it might be used separately. It
must implement keep-alive, but it does not yet.
You must not put it to ConnMan by yourself (instead, use start()) and
you must call close() after you've finished using this request.
You can either work with it in callback, or wait() and simply use its
methods (check it's success() and then, for example, use text()). Like
this:
```
Networking::SessionRequest *rq = new Networking::SessionRequest(url);
rq->startAndWait();
if (rq->success())
warning("HTTP GET: %s", rq->text());
rq->close();
```
