rgw: clean up shutdown signaling

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: allow multiple frontends of the same framework

it is possible to set up mongoose on multiple ports, e.g.,

  rgw frontends = mongoose:80, mongoose:8080

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