CBQE-0: changes in execute_cbstats to use "allocator" stats

Change-Id: Ic09dd4722cc818d7d1ef15ce5a949808746058dd
Reviewed-on: http://review.couchbase.org/32728
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>