Use HTTP1.1: we get persistent connections for free

Hi,

Currently rospy creates a new socket for every single request (eg. getParam call). Since sockets stay around for a while in CLOSE_WAIT state, this can become a serious problem.

Python's xmlrpc actually supports persistent connections. All that's needed is configuring SimpleXMLRPCRequestHandler to use HTTP 1.1 (vs. 1.0) and it'll automatically keep connections open and re-use them for subsequent calls.

With this you go from:
```bash
$ sudo netstat -atpn | grep 11311 | wc -l
1434
$ python -c "from rosmaster import util; s = util.xmlrpcapi('http://localhost:11311'); [s.getParam('foo', 'bar') for i in range(1000)]"
$ sudo netstat -atpn | grep 11311 | wc -l
2449
```

to:

```bash
$ sudo netstat -atpn | grep 11311 | wc -l
1728
$ python -c "from rosmaster import util; s = util.xmlrpcapi('http://localhost:11311'); [s.getParam('foo', 'bar') for i in range(1000)]"
siegfriedgevatter@thailand:svn:
$ sudo netstat -atpn | grep 11311 | wc -l
1732
```