rgw: reuse fcgx connection structure

Fixes: #10194

Reuse the fcgx connection structure. It forces FCGX_Accept_r() freeing
up or reusing the connection if possible. This is needed for certain
fastcgi module (mod-proxy-fcgi) that sets the FCGI_KEEP_CONN, which
prevents the connection from being closed when calling FCGX_Finish_r().

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