MB-11351 make sure that replication_manager:get_incoming_replication_map

... does not block if the replication_manager is blocked

Change-Id: I47ad856dc81176cefd2ea415974aeae18a66ea5d
Reviewed-on: http://review.couchbase.org/38120
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>