MB-9898: Make using streams lock free in upr consumer

This change greatly reduces the locking overhead in the upr
consumer class by making stream accesses lock free.

Change-Id: I6d0737f2f328f03cb000d39850aca2a4b90ad389
Reviewed-on: http://review.couchbase.org/35373
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>