Callback driven API for config_listener behaviour

Callback driven API for config_listener behaviour

This implementaion replaces a mix of message driven and callback driven
API for config_listener. In particular it replaces gen_event_EXIT
message with a call to Module:handle_config_stop(Pid, Reason, State).
This fixes the problem of using config:listen_for_changes in supervisor
context where there is no way to handle arbitrary messages.

COUCHDB-2561