rgw: rename data receive callbacks in swift token revocation

Fixes: #5921
As part of the work that was made for dumpling, the http
client in-data callback was renamed in order to avoid confusion.
However, we missed the rename in a couple of places, which this
patch amend.

Reported-by: Roald van Loon <roaldvanloon@gmail.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: rename read_header() to receive_header() where needed

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

Reviewed-by: Sage Weil <sage@inktank.com>
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.

Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>