make menelaus_cbauth gen_server to avoid blocking ns_config_events
make menelaus_cbauth gen_server to avoid blocking ns_config_events

... during revrpc call

Change-Id: Ibedc07f1457c38b637c662a723eb913e57098d9e
Reviewed-on: http://review.couchbase.org/45187
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

mschoch/ns_server,mschoch/ns_server,nimishzynga/ns_server,membase/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,mschoch/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,ceejatec/ns_server,ceejatec/ns_server,mschoch/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,mschoch/ns_server,membase/ns_server,mschoch/ns_server,ceejatec/ns_server