CBQE-2290: add timeout for getting stats

fix test hanging

Change-Id: I58c099b57196342ed394ac00940e7bb76456975a
Reviewed-on: http://review.couchbase.org/38677
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>