MB-15283 do not crash menelaus_cbauth if json_rpc_connection exited
MB-15283 do not crash menelaus_cbauth if json_rpc_connection exited

...during the call.

the cbq-engine was restarted. new json_rpc_connection was created which
killed the old one with new_instance_created. this caused menelaus_cbauth
to crash and all other cbauth connections to be recreated

Change-Id: I7623173cb7233c049a4a765365b4f899e553215a
Reviewed-on: http://review.couchbase.org/52040
Tested-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

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