Better connection sharing by using only one curl object.
* bzrlib/transport/http/_pycurl.py: (PyCurlTransport.__init__, PyCurlTransport.has, PyCurlTransport._get_full): Use only one curl object. (PyCurlTransport._get_ranged): Use only one curl object. Specify 'Range' as an additional header. (PyCurlTransport._curl_perform): Take additional headers into account. (PyCurlTransport._set_curl_options): Leave _perform do the header setting. (PyCurlTransport._curl_perform): Set all headers just before issuing the request.