rgw: rename read_header() to receive_header() where needed

The same as with #5921, fixing a missed interface change.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: make RGWHTTPClient callback methods pure virtual

This will make it easier to catch interface changes, like the
ones that triggered #5921.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>