made pull_plug synchronous so that bug 1555 is fixed
made pull_plug synchronous so that bug 1555 is fixed

In add_node we send otp cookie to other node right after
engage_cluster which, potentially, call ns_server_sup:pull_plug (via
ns_cluster:rename_node). And doing pull_plug asynchronously causes otp
cookie race.

Change-Id: I3fa4ac20448c85dabdf533801a830ebeb58ed07f
Reviewed-on: http://review.northscale.com:8080/856
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>

pavel-blagodov/ns_server,daverigby/ns_server,nimishzynga/ns_server,daverigby/ns_server,membase/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,membase/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,vzasade/ns_server,ceejatec/ns_server,vzasade/ns_server,mschoch/ns_server,mschoch/ns_server,ingenthr/ns_server,ingenthr/ns_server,membase/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,ingenthr/ns_server,daverigby/ns_server,daverigby/ns_server,vzasade/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,ceejatec/ns_server,ingenthr/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,membase/ns_server,membase/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,mschoch/ns_server,nimishzynga/ns_server