connection-read-handler: prevent processing stale read events

We don't want to hear about read events from our base fetch
connection anymore when the base fetch handler indicated it was
done.

One concrete problem this solved for me was that SPDY combined with
IPRO would segfault and/or pass on 5xx/404 responses from ipro lookup
fetches to the browser, next to alerts about r->count being zero
in nginx's error.log

Might fix https://github.com/pagespeed/ngx_pagespeed/issues/788
Fixes https://github.com/pagespeed/ngx_pagespeed/issues/792