MB-10465 eliminate races between producer and consumer sockets

... during takeover

1. obtain stream opaque from stream_req because it for sure will
   come earlier than set_vbucket_state
2. change set_vbucket_state from cast to call to guarantee that
   set_vbucket_state won't be proxied on producer before the consumer
   changed its state

Change-Id: I425373777aea5424c2d6ec172608c922d865b434
Reviewed-on: http://review.couchbase.org/34881
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>