rgw: only fetch cors info when needed

rgw: only fetch cors info when needed

Fixes: #5831
This commit moves around the cors handling code. Beforehand
we were unnecessarily reading the cors headers for every
request whether that was needed or not. Moved that code to
be only called when needed. While at it, cleaned up the
layering a bit so that not to mix S3 specific code with
the generic functionality (except for debugging).

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>