avoid xdc_rep_manager call for xdcr stats/errors gathering

Because we can do it directly without any extra serialization implied
by gen_server:call.

Change-Id: Ib9b0f0feb9d49320cf6c665fe680ce839d79282c
Reviewed-on: http://review.couchbase.org/40090
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>