Add UPR_OPEN_PRODUCER flag

Change-Id: I2afd081f515dbcb5bedbf2c8f4aa3ef251176546
Reviewed-on: http://review.couchbase.org/29833
Tested-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>