rgw: optionally call FCGX_Free() on the fcgi connection

Fixes: #10194

A new configurable controls this behavior. This forces disconnection of
the fcgi connection when done with the request.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
Merge pull request #3124 from ceph/wip-10194-firefly

rgw: optionally call FCGX_Free() on the fcgi connection

Reviewed-by: Sage Weil <sage@redhat.com>